As Zarnick said, don't expand and build up too fast.  If you're not already, try colonizing only the highest quality planets.  Use the Federalist political party - they give a 20% economic boost.  Grab every economic mining resource you can and build them up to the max.  Increase your tax paying population by building farms and using morale resources and researching morale improving techs.  I normally play with the Super Breeder ability, as that shortens the time that a new colony takes to break even (as long as your tax rate isn't set so high that approval isn't 100% even on a new colony).  

When I first started playing I took the advice I had read in the few guides there are and stayed away from placing farms on farm bonus tiles and to refrain from developing farming beyond Basic Farming. Now, every planet above PQ6 gets a farm, and if I have an entertainment bonus tile as well as a farm bonus tile, then I will build up to lvl 2 farming on the farm bonus.

What I end up with is around 50% of my planets have a pop of 10bn, another 25% will be in the 12-18bn range, and the remainder (usually the under PQ 6 planets) will be at 6bn. The net effect is to have taxes and tourism cover all of my expenses at the least, and have trade be over and above that. And yes, stock markets are cumulative. I develop most planets with this ratio: 1 entertainment building for every farm, 2x as many factories as research, and 1/2 as many stock markets as factories/research. So a typical PQ12 planet with no bonus tiles might look like this: colony tile, starport, farm, entertainment, four factories, two research, two stock markets. Bonus tiles can alter things a bit depending on what they are. There are, of course, many different ways to develop, this is just my way which seems to work for me.

When the inevitable happens towards the end of a game (whatever civs are left ally against you), my economy can stand on its own, while my enemy's econs go straight into the toilet because their trade with me was a hefty chunk of their income (and the first thing I target is their merchant fleet, dropping their trade income to 0).

As to the race relations, someone could be paying another civ to go to war with you. Take a look at the relations screen to see which factor +/- in your relations suddenly changed. 

@Zarnick

How high of production should I be at?  Once im up and going, im normally at or around 75-85 depending on the galactic economy.

@ Publius of NV

I already look toward the biggest and best first.  However once other races start encroaching, I grab the other worlds within my borders, as I have found, when I don't im almost always in for a fight in short order.  I also usually end up being Neutral, if that matters.

I grab all the resource nodes I can get, as far as mining asteroids, I grab those quickly as well, they seem to help a good amount too.  Also, is Merchant Trading Centers a racial exclusive skill or can I research it as well (I dont have the tech tree memorized).

Also, what is your tax rate/approval normally, as a ballpark guestimate?

@Klaatoo

Interesting basic build idea.  What do you do for your heavy production needs?  For instance you are needing you 2 biggest chassis cap ships built in reasonable time frames, do you have worlds who are solely built for that purpose?

@dnzrx

Good point, I will keep that in mind

In general.

Forgive the ignorance, I found the downloadable 500 turn demo, loved it, bought it and tested the depth of the water with both feet, so to speak.  I learned by getting my @$$ kicked a few times, I always prefer to learn by my own mistakes rather than read a guide, but I am at the point where I guess more than anything else, I need clarification.

Does a factory assist with production on a planet in general?  For instance, I build my factory, it will then help get my economy buildings and other structures up faster?  If not, I have really misunderstood a few things.

What do I do to grow Tourism?  It seems to grow as I have more planets, and with my Influence stations (which I tend to build alot of.... worked well for me thus far).

Is it really that beneficial in the long run, to take smaller planets just to laod them up on economy boosting buildings?

Mid game, once i am established, I usually colonizedall the 5-8 building planets and from there I make all of the research, or alot of them and the rest econ.  Is that not a good idea?  And if not, why? 

I dont remember what my racial picks are atm, I amde a race I liked and ran with it, I figured I would have to revisit it, I know I run Universalists, as it has good all around buffs, didn't remember which other option I took super breeder or whatnot.  I know I was leaning towards heavy research, as that was how I dominated in MOO2, I know this isn't MOO2, but that was my BS space game I knew, so I tried to stay with a known good idea and learn the new game rm there.

  If you are at 60% production, you will have the same output at 100% production with 60% of your factories.  You could build market places instead of some of those factories.

  I don't know any reason for having less than 100% production except when I suddenly lose a large source of income and need to keep my treasury in credit for upgrading starbases or rush-buying.

Tourism depends on your influence so either grow your population or research technologies that increase your influence. Personally, unless there's this event that quintuples tourism income, I've rarely found tourism income to be significant enough.

Regarding your question on a race's stance towards you: you should be able to see the positive and negative elements in your relation when looking at the race's details.
Examples of positive elements are: trade, economic / research / overall alliance, same ethical alignment, your military rating being significantly higher than theirs, high diplomacy, ...
Examples of negative elements: previous wars with them, different ethical alignment, having borders with them, your military being weaker than theirs, them distrusting you if you're being aggressive towards other races, ...

For your overall gameplay I've listed some recommendations below. While I'm definitely not the best or most experienced GalCiv II player out there, I do well on difficulty levels ranging from painful to crippling depending on the race and galaxy size.

1) First turn: set your production slider to 100% and lower the tax slider to get to 100% approval rating (for some races you may need to settle for 76% if you can't reach 100% approval).
Having 100% approval on a planet gives a bonus to population growth on that planet. The same goes for 76% approval but the bonus will be smaller.
A rule of thumb is that once my cash reserve is only sufficient to last 30-40 more weeks at the current spending level, I'll increase taxes at the expense of my approval rating. As long as my weekly balance is too negative, I don't spend anything on espionage yet.
I never lower my production slider below 100% unless I made an error of judgement and I'm getting too close to being 500bc in debt (if your debt is higher than that, your production is set to zero automatically until you're back above -500 bc)

2) In galaxies up to medium size, only build on 2-3 planets early on in the game. Colonize other planets with decent quality but don't build anything on them yet to reduce your expenses.
I usually get some production (factories) on my homeworld, add 1-2 morale buildings and as soon as possible an economic capital + some econ buildings (banks, trade centers, ...)
My second planet will be dedicated to the production of colony ships. Crucial here is to rush-buy some factories and a starport relatively early.
My third planet will be mostly dedicated to research (labs) though it's usually best to start with a factory as it increases your building speed.
For larger galaxies, I'll also build factories and a starport on a more distant planet as this will improve my colonization efforts.

3) Research: go for some basic techs first, the ones that give you bonuses to social / military production, morale, economy. Some speed bonus also helps to be able to grab planets before the AI reaches them.
Trade is also a good early tech to research as it allows you to build an economic capital.
Trade technologies and influence points (never more than 999 at a time) for other technologies or cash, especially with minor races. It's best never to trade any technologies that give your opponents a diplomatic bonus since that will hamper your future interactions with them.

4) Military: while I usually want to be the first to have military ships, I don't try to outclass the other ratings too much in military rating until my economy is sustainable (either running a small loss each turn or being in the green)... or until I want to go on the offensive.

Hoping this helps a little...

Merchant Trade Centers are a racial exclusive to the Iconians.  I normally play as a custom race with the Terran tech tree, so whenever I can I try to trade with the Iconians for it. 

Since I'm playing as a custom race with super breeder ability, I try to keep my tax rates low to build up my population quickly.  I normally start out at a tax rate of 49%, and then when I have a fair number of colonies that could grow faster if their approval was at 100%, I drop it to 39%.  This is usually after I've been able to build an economic capitol and have established a few trade routes.  The 39% tax rate pretty much makes all newly colonized planets have an approval of 100% until they are close to their starting max population (before any farms).  I tend to end up with most planets having a population of 15B, with the home planet and any captured home planets at 16B, and any planet with class 20 or higher 18B (with the food distribution center).  I do not build farms on the food enhancement tiles.

I don't pretend to be the best GC2 player in the world, but this seems to work for me.  The only time I'm close to hurting for cash is if I haven't been able to claim any economic resources and haven't yet been able to build an economic capitol.

There are many others who keep their tax rates much higher than mine, but that doesn't play well with the super breeder ability.

A handy tip if you're wondering whether to concentrate on building Embassies or Market Centres on a particular planet for tax or tourism income - check out the influence points gained from the planet itself by hovering over the ip indicator.  If it's 10 or more then, even with no other influence bonus, two embassies will net you at least 3 additional ips.  Two Cultural Exchange Centres would net you at least 5 additional ips.

Having your taxes higher than 35% is not an issue, it all depends on how this affects morale on each of your planets.
In case of morale dropping below 46% there won't be any population growth on that planet anymore. If you go even more extreme (here I don't remember the cut-off point, roughly around 20%) your population even starts to shrink.

I would recommend to never decrease your production below 100% because then you risk falling behind compared to the AIs. Increasing taxes and trading techs / influence points for cash should keep you afloat for much longer than 10 turns. It should be possible to stay above -500 bc throughout the entire game without having to lower your production. However, playing around with the military / social / research sliders can make quite a difference on your costs if you're in a tight spot.

Well, I thought production (employment?) slows down all areas while playing with the sliders slows down or speeds up specific areas. Is this not the case?

So if you have 24/24/24 ability with 100/33/33/34 sliders and no military project but ongoing social and research the social costs 7 and the research costs 8 (total 15). With 100/0/66/34 social costs 15, research 8 (total 23), with 100/100/0/0 neither social nor research costs (or progresses).

1 bc = 1 tp = 1mp is correct.
There is an impact from the "bonus production / research" (though I have no idea how that is calculated) but the major difference is coming from planets where you don't have activity in all 3 areas military / social / research like Antibor indicates. If I have a planet where I'm not building any ships or buildings, funding in those 2 areas will not be used, only the research money will be spent.

I think there's also an influence from focus: imagine many of your planets have a focus on research while they have military and social production ongoing. The more you go towards 0/0/100, the lower your spending should become since you're moving X points in social / military production to only 80% of X points in research.

The percentage chosen with major production slider is indeed multiplied with the 3 sliders below it. Setting the overall production to 50% will reduce the cost of your social, military production and research by half (and the output as well).