My car wont go into gear, it starts but wont stay in gear?

I have an 2000 automatic Buick Century, and the car starts fine but once started it wont stay in gear. It seems like it is slipping gears and just wont catch on any of them if that makes sense. Any ideas about what may cause that? And what I should check first? I am trying to postpone taking it into the shop due to lack of funds. So any help would be greatly appreciated.