Step one is to go through Facebook's privacy functions (extensive) and start changing settings.
Privacy settings can be found in that down-arrow to the right of the word "Home." For almost everything, I have it set so that only "friends" can see most of my stuff. When in doubt, err on the side of tighter security.
You can actually globally change your past posts to a tighter security level, which is kind of nice.
A lot of applications, games, and things like that accessed through Facebook have a screen come up where they would like access to some of your data before they let you through. Thanks, but no thanks.
Security settings for things like Google and other widely-used programs also need to be tweaked if you sign on with them.