I'm mortgage illiterate help?

I've been interested in buying a house, I currently rent an apartment. What are the basic facts of a mortgage? How much do you put down? Is there a minimum you can put down or does it have to be a certain amount? What would an average monthly mortgage be? How do brokers/realtors play a role? The more descriptive the better thanks. One more question, I have average credit but my g/f has awsome credit...would she have a better chance of getting a house. PS. She doesn't make much but I would obviously help her if that's the case. thanks again