Ok, so probably a good place to start is to learn a handful of songs. Essentially, whatever you practice, is what your going to get :p

So if your looking to entertain small groups of people with simply your singing and guitar (assuming your wanting to sing and play) than thats what you need to practice. Learn some songs through tabs, and just start playing and singing along with the albums.

Eventually, you'll get to a point where you will start writing your own stuff.

As for theory, this website is a good place to start.