First, you'll need to talk to the owner to see if he'll be willing to provide you with seller financing. If he's ok with that, then you'll just need an attorney to draft up the terms of the sale. If he's not ok, then you'll just have to start looking for another place to rent when the owner sells.
It really depends on your definition of "bad credit". talk to a reputable lender(and I can give you names). Most owner financed deals, the owner wants quite a bit of down payment. Let me know if you have some more questions.