You need to know both your credit history and your credit score (sometimes called a Beacon score or a FICO score). Both your payment history and your credit score will determine what type of better car loan you qualify for - prime, near-prime or subprime (sometimes called a bad credit auto loan).
Research the kind of vehicle you're interested in. You should determine what you'll need in terms of vehicle size and performance, as well as how this vehicle will fit into your overall budget. If possible, you should choose at least two different models and test drive both, before you plan on applying for an auto loan.
Option out the vehicle the way you would like it and then determine the suggested retail price (MSRP) as well as the dealer invoice price. You should also check to see if there are any customer cash or rebate programs that could further reduce the selling price and the amount of the auto loan. If you're considering trading in your current vehicle, you should also research its market value. Many auto dealerships for bad credit can offer you incentives on new cars.
When it comes time to negotiate, it's always a good idea to keep the new car and trade-in negotiations separate. Keep in mind that the bottom line in any transaction involving a trade-in is the difference amount. The lower the difference amount, the lower the monthly auto loan payment will be.
Finally, and most importantly, don't take your "eye off the ball" until you've completed the last part, which is the paperwork process. Keep in mind that you are not signing a contract with the auto dealership for bad credit; the dealer is merely acting as an agent for the lender. If something you negotiated during the buying process is not in the contract or on the buyers order, than it doesn't exist. And since there is no cooling off period after signing the auto loan agreement, this means that once the papers are signed, the deal is final. For this reason you should take the time to read over all the paperwork before you sign the auto loan agreement. Make sure everything contained in the paperwork matches the terms that you agreed to and, as always, get a copy of everything that you signed at the time of delivery before you leave the auto dealership for bad credit.