In beginning be careful on colonizing planets in systems that contain AI player planets, they tend to take a dislike to you and this could affect your treaties and/or their attacking you, at least in beginning until you get stronger, also mines, mines, mines, in the beginning. research m ines with warheads and put some up over your planets, this should protect them in early game, then do whatever your style is, research, colonizing, weapons, ships, whatever, choice is yours everyone has different tastes, but in beginning be sure you put defense up over your planets or you will be sol against aggressive ai player. BUT remember, this works for ai players, when playing human a large number of things change and things you do well against AI won't always work against human player, good luck, have fun, enjoy...
be careful, be very very careful
look to the sky
its "THEM"
just some ideas mac