[dmm]

There's a strategy guide that several people have been working on. Take a look.

My 2 cents:
1) Learn the game on turn-based, with low tech and low research cost and no AI bonus, and use tactical combat. It's easier.
1a)Also, make your race friendly and peaceful. The AI will be more accomodating, and it turns out that peaceful races don't mind battles much, as long as you win.
2) Don't make treaties with anyone stronger than you. They'll abuse you -- colonize planets in your home system, etc.
3) Unless they're a LOT stronger than you, in which case you should grovel to play for time.
4) When learning the game, use emergency build to your advantage. Once you get good, you can eschew this sleazy tactic.
5) As in all wars (game, real, economic, political, social, or otherwise): Concentrate your fire power. Try to fight battles with the odds highly in your favor. Let the enemy destroy himself against your fixed defenses before engaging him in open battle.
6) Colonize larger worlds and breathable worlds preferentially, but don't spread yourself too thin in doing so. Don't go more than one system away at first. In the early game, junky worlds in your home system are more useful than paradises 5 turns away. (The 5 turns you spent moving a colony ship to Eden would have been enough time to build a shipyard on nearby Inferno Moon.)