The worse your credit the better your luck would be at a dealership. They have a bunch of banks they can go to for financing, but, they'll screw you with a crazy high interest rate and/or extended term (5yr on a used car). So, it is in your best interest (no pun intended) to go to a bank/credit union and apply before shopping.
Keep in mind that they will all only finance the loan value of the vehicle, so, you have to come up with the difference between that and the sell price, and then anything above that will be what helps you. So, if you can find a good deal on a used car (selling close to or below the loan value) you'll be in much better shape.
----- A long story about another option:
A family member of mine had an ordeal recently, she has OK credit. She decided she needed a new car and bought a 4yr old civic (that was in great shape w/ low miles, but still 4yrs old) from a dealer who was going to have her finance it for 4-5 yrs so she could make the monthly payments. Turns out they let her drive it away w/o securing the financing so on Monday she got a call saying she needed to come back down w/ some significant extra cash to get her financing. She freaked because she didn't have the extra cash.
I was able to get involved at that point. I read the contract and basically is said if she didn't get the financing she had to return the car and they had to refund her money / down payment. (Fortunately she did not do a trade in -- or we would have had a bunch of headaches trying to get that back). Bottom line: She did not have to pay anything more, unless she wanted to. I told her she could get a new civic for the same payment but it'd be 6yr loan (which would still be a good deal vs the 4yr old car with a 4-5 yr loan). At the end the new car would be 6yrs old v 8-9yrs the other way.
So we returned the used civic and walked next door to the new car dept. We negotiated for a 2002 EX and did the paperwork again. A couple days later my sister called saying that she didn't get approved for a 6yr loan but they had secured her on a 5yr loan (that we hadn't agreed to). Turns out the 6yr car loans are only valid on new cars >$25K -- at least in this case. Well, she could return the car and get the money back OR make higher payments. We decided we'd shop elsewhere at that point and return the car. Of course the dealer was amazingly now able to sell the car at cost which suddenly made it affordable again.