You could start with the Beginner's Strategy Guide. A lot of things have changed since it was written, but most of the basics still hold true.
Play the campaign scenarios; you don't need to know everything to play them, but you learn enough about GalCiv2.
I don't think that's a good idea. Unlike in other games, the campaign expects of you to know the basics already. It's more for advanced players, who are looking for something different for a change. The sandbox is a much better place to learn how to play.