Depending on the victory conditions you have set (tech, cultural, etc...) you can win in one of five ways.
But the most fun is militarily: colonize every planet you can find with a PQ of 15 or more, build social projects to improve their value and ability to produce, research like mad, then send your heaviest ships (dreadnoughts work wonders in packs of 10) and carpet-bomb the other races back to the stone age. Once their orbiting ships are eliminated, you just send a dreadnought to the planet, and invade.
I don't know about you but I like to keep a squadron of my strongest ships by the planet I'm going to invade so I can take out ships going for my transports. When it gets there I use the Tidal Disruptors. They cost 700 BC and as far as I can see the planet only looses one or two improvements.