Welcome to the GalCiv2 community!
For good information without spoilers I recommend:
The Galactic Civilizations 2 Wiki
It's gradually being updated to include the new stuff in Twilight of the Arnor. It's maintained by players.
It contains a link back here to the Begginer's Strategy Guide. This guide was designed for use in Dread Lords and should work fine for Dark Avatar as well. Starting off in Dread Lords then working your way through the expansions may give you the best "bang for your buck".
There are threads around these boards for advanced strategies, but if you're indeed new to this game and/or genre, I'd recommend against reading them until you're gotten bored and need something new to try. That's the approach I took, and helped increase the longevity of the game for me.