Some important things to remember, for the medium-large range of galaxies with "typical" settings.
COLONY RUSH:
Early in the game it's important to colonize as many decent planets as possible, the good ol' colony rush (ignore PQ 1 and probably PQ 2 planets... probably useless the AI will be ignoring them too, so no need to grab them right now). Get some more factories on your homeworld and send out some colony ships. You want to send them *outward first* to get worlds before the AI does. There will always be a decent planet right in your home system, do NOT send your first colony ship to it. Send your third or fourth colony ship to it, because it will be a while before any AI tries to colonize that planet. Use the minimap to send your ships toward star clusters. If you're toward an edge of the map, send your ships toward the middle first - the point is to be the first to reach systems that are likely to be contested. Stars between you and the edge of the map are not likely the AI's first target, so you can send a ship toward them later.
DEFICIT SPENDING:
Do NOT spend all of your early cash rush buying things. If you have anomalies on (most people do), set your survey ship to auto-survey. This is its most valuable use. Rush buy things only if you have lots of money in your treasury, say 3000-4000 BC. Rush buying one or two factories may not do as much as you think unless you build them on bonus tiles - your homeworld's colony building already produces as much as six basic factories or four standard factories.
Use the rest to sustain a massive deficit. Lower your taxes until you get 100% approval. Keep this going as long as you can, to grow the population on all your planets at double rate. You will often be losing 100+ BC per turn. If you have a lot of cash anomalies you can keep deficit spending going for ages. Otherwise, you'll otherwise have to set taxes to about 49%.
TAXES AND APPROVAL
Now's the time to learn the relation between taxes and approval. It's not straightforward, there are "cutoff points".
Basically, for any form of government you want to avoid rebellion or losing elections, and high population growth is nice. Above 60% approval you're not likely to lose an election (I think imperial governments may be able to go lower). At 75% you get a significant bonus to population growth. At 100% you get double population growth. But basically, if population growth isn't a huge priority then approval above 60% is basically OK.
Up until the 30-40% range, increasing tax point by one point decreases approval by roughly one point. But as you increase tax you will see some *sudden* drops in approval, usually at numbers that are multiples of 10. One of the biggest is at 50%. This is why I tend to keep my taxes at 49% until I have quite a few morale-boosting technologies.
UNDERDEVELOPING NEW COLONIES:
A new colony is a money sink. At the beginning of the game, you're short of cash. So... it's often best not to try and do much with a new colony. Put a few factories in the build queue (don't rush buy anything) so that it's ready to be developed when you have the money, but consider just leaving them idle. Definitely do not build other stuff until A) the planet has enough population to start generating taxes or
your empire has enough money that you need more space to build factories and labs, and the space might as well be on this colony.
In the early game you really have to be careful with your money. A lot of your colonizing is just "planting flags" very quickly to keep the AI from taking the worlds. You can't actually *use* the colonies quite that fast. So you develop your homeworld and one or two of the best worlds you've found, and then you build up other worlds as you have the money to pay for them. Otherwise you will have lots of planets full of buildings that you can't pay the maintenance for.
MAXIMIZING YOUR INCOME:
This game has some interesting formulas for making money. The amount a planet makes depends on the tax rate, and how much you can tax depends on morale. What's taxed on a planet is its population (not industry/research output), so as the population goes up the tax goes up. But as the population goes up, morale goes DOWN. To keep morale up you have to build morale-boosting structures, which takes up space that you could have used for something else. Like, for example, stock exchanges that would boost your taxes!
The way it works out, the ideal population seems to me to be somewhere in the range of 12-15 billion (it depends on planet quality and tech level). Not nearly as high as you might think.
First reason: tax rate is proportional to the *square root* of population. This means that doubling the planet's population only results in a 50% increase in tax rate. So a pop 24 planet only makes 50% more than a pop 12, and a pop 48 would only make 2.25 times as much.
Second reason: there is a *big* dropoff of approval that starts roughly around population 20 and reaches a huge peak around pop 25. There is just no reason to exceed pop 18 or so at all, morale goes too low.
So to make a lot of money you find a pretty high PQ planet (minimum PQ 10). Build farms to get the population to 12 billion, or around 15 if it's late game and PQ is higher. Build one morale building, or 2 for population 15. Then you basically fill it with trade centers/banks/stock exchanges.
Obviously, your economic capitol should go on such a world. Do NOT build the economic capitol on your homeworld if you don't plan to make your homeworld into an economic world in the long term.
However, unless your homeworld has lots of manufacturing bonus tiles or something, I strongly recommend making it into more and more of an econ world as the game progresses. The economic capital can be build *early* in the game because Trade is not a very expensive tech to research, and it will greatly improve your finances. Your homeworld has the population and PQ to make a good econ world. And in most cases it will be relatively far from the front line as the game progresses.
PLANET SPECIALIZATION:
The section above suggests specialized economic worlds. To get a money-maker planet (other than on your homeworld which starts with a higher base population) you might start with two farms, a morale structure, and a minimum of four or five econ structures (banks etc). There's no point spending the space on all that population boosting if you can't also put in the econ boosting structures to roughly double your output.
Add in your initial capitol and two factories to leave around for upgrades and that's 10-11 tiles taken up right there. Unless you find one of those rare worlds with ridiculously high PQ, your economic planets should be fairly specialized in what they do. They might have a sideline in building smaller ships or galactic achievements or whatever, but they should basically be specialized money-makers.
Which brings us to the rest of the planets... they have no reason to be specialized money-makers, and therefore they have no reason to exceed pop 6. In fact I often mine their population for transports, dropping it down to 2-3 a lot of the time (drop it further and it will grow back sloooowly).
Your other planets will be the money spenders, research and industrial worlds. You can split them between research and industry, or you can have all-research and all-industry worlds. At least a few worlds *should* be purely specialized. Your technological capitol structure, for example, is precious. Build it on a relatively high PQ world. I'd consider anything less than PQ 10 a waste unless you are really strapped for planets... you want it to apply to at least 6 labs even before you get to advanced terraforming techs, or the bonus-tile-boosted equivalent of 6 labs. That structure is a prize... it doubles the research output of a planet while only increasing the cost by 100%. Anyway, stuff that planet with labs and just enough factories to upgrade them.
It's really all about maximizing use of space. A good research world should have a research coordination center, but you need 4+ labs for it to be worthwhile. A good manufacturing world needs a starport, and it should have an antimatter power plant, but now you need at least 4 factories for that to be worthwhile (and only one world can get the manufacturing capitol, so it should have as many factories as possible). If you're working with worlds that are "only" PQ 8-10, then generally they make good manufacturing or research specialist worlds.
For worlds with very low PQ, I give them some factories then I either build a starport and use them for emergency buying ships, or put them "on hold" for better terraforming techs, or I use the planet as a dumping ground for galactic achievements.
BUILD-AND-REPLACE FACTORIES:
An often overlooked fact is that you can "upgrade" any structure into any other structure. What this basically means is you build something on top of something else, and the previous structure doesn't stop working until the instant it's replaced.
This is useful for quickly developing new colonies. Basically, you can build more factories than you'll ultimately need (especially if the world will eventually be a research or econ world), and especially you can build *older* factory designs which are faster and more efficient to build. The classic Basic Factory and Factory designs are the fastest to build for how much they produce (Factory just has a slight maintenance cost and takes up less space). So what you can do is practically fill up a world with these, then use them to "upgrade" to more advanced structures right on top of themselves.
THE LAST MINUTE MILITARY:
There will come a point in the game where the AIs will have finally build up a bit of real military, and start to look for easy pickings. If you're weak at that point you will suddenly see them ALL start to threaten you and/or declare war on you at once.
You want to be ready before that point - by having established some diplomacy, and by having some military techs and ships ready. And you want to watch for it coming. You can look for relations going downhill and one of the "minuses" on the foreign relations screen being military strength, and other indicators are them developing planetary invasion tech (this means that if they declare war, they're actually a real threat rather than a nuisance).
But until that point is nearing, don't build a military unless you really want to beat the AI to the bunch. Research the core techs that will build up your economy, industry, and so forth. Some that new players may forget include trade (economic capitol), xeno ethics (instant +15 morale), and the one that gives you harmony crystals.
