How do I except a mission

You don't. Just fulfill it, or let it sit there.

I am new to this , how do i fulfill the mission

Do what it says. It will generally ask you to destroy ships/civilian structures (Logistical slot using structures)/tactical structures (Tactical slot using structures) or will ask for a certain amount of resources. Go to the diplomacy screen and that empire's tab for the buttons to do those.

The tribute missions are a good way to make early allies with the AI. Later on, Keep your tactics aggressive and you will make allies pretty much automatically. Someone will always hate someone else, and be happy to have you kill them.

If you ignore all the missions, you will be the most hated person against all the AI players.

I find it real hard in the middle of the mid-late game of multi star system map to keep allies. Mainly because I am eradicating a threat from my starting star system and I keep getting freakin requests from my allies to attack there enemies that are in their solar system. FREAKIN STOP ASKIN ME, I AM TRYING TO SECURE MY SOLAR SYSTEM FIRST!! And so many of my peace treaties fall apart about then and then I have to quickly get a cease fire going or face fighting a war with very stretched out forces.

Any target that you're given a mission for is within 2 jumps of some part of your territory, that's how the missions are designed. Of course, stars count as a jump as well

It might help to research the happiness techs. They decrease the penalty and increase happiness gain, so for any mission you do you can fail 4 or so (that's 2 hours of gameplay) and it will approximately even out. It's even better if you manage to get to 100 before completing some becomes an issue, because the AI won't break a cease fire until you drop below 35.. and that's a long long time if you only lose 7 happiness for each failed mission

Those are always a priority for me. They're extremely useful in unlocked games.

Do all races have the quote unquote Happiness tech?

TEC gets Persuasive Diplomats.
Advent get Induced Leniency.
Vasari don't get one.

The effects are a bit different for each, but are similar.

I was wondering since I'm playing a match as the Vasari and I didn't notice one. Buuut now that I have a pretty good understanding of the tech trees I don't normally read what each tech does and I just research things that I find important and then I just click on other techs when I dont have anything important to research at the moment.