This is the official site for Galactic Battlegrounds. You can hit their main site, http://www.lucasarts.com, for the other games. There are other unofficial sites out there that will all give you variations on that same info.
Also, Battlegrounds is using the same engine as "Age of Empires II", so if you're familiar with that game then that might help you out.
You might be able to pick up a strategy guide at Best Buy or a similar store.