Build a mighty fleet!
Settle the galaxy!
Crush your opposition!
Seriously, this is a very broad question.
General hints:
-Switch "Invaders" off when creating a game, or they might come in after a year or two and whipe the floor with you
(you can switch them on later)
-Colonize a planet in your home system, to get civillian commerce started
-Build some defenses on your homeworld/in your home system (Planetary Defense Centers (PDCs) work well for this) to keep your people feel all well and happy.
-Keep an eye on your resources (TN minerals) and find a new source BEFORE you run out in your home system
-Be not afraid to ask stupid questions. We have all been there