Financing a Car with bad credit.

There are a lot of places out there that will loan money to people with er, "challenged" credit. All you need to do is show you have a job, a bank account, and a couple of personal references.
Now, keep in mind you ARE going to pay huge interest rates. Most of the time, your payments are set up weekly. Be warned though, if you miss, or are late on even a single payment, you car will likely be gone with any down payment you may have put on it. And they usually require a substantial down payment.
There are many decent dealerships out there that will finance just about anyone, but you ARE going to pay through the nose, and the vehicle is going to be one that is way down the list of "desired" vehicles. Usually they are lease cars that have ton of miles on them for the year they are, trade-ins that better or higher class dealers do not want, etc. You are going to end up paying over twice what the vehicle is actually worth, or what someone with cash or good credit could walk up and buy it for.

Don't go to Big Ed's used cars for people who have bad credit, you know what I am talking about, the little car lot on the corner with a storage shed remodeled into a office. Unless you want a real piece of crap, for a ton of money, and a couple of broken legs if you miss a payement. There are a lot of larger respectable dealers who have programs for people with bad credit, find one them.

If you have old unpaid debt, pay it off, you don't need to be financing a car. if you absolutely cannot and need to go the "i'm a bum" route, then wait for it to fall off your credit reports, being sure to NEVER make any more payments on it (or else you'll reset the clock on the statute of limitations). When it is gone from the reports AND the SOL has run out, reply to the next message from the collector by sending a certified letter apprising the collector that the SOL has run out (many examples online).
Get one (regular unsecured) credit card. (If you can't, then start with a secured one). Each month for 6 months, charge between 1/6 and 1/4 of the limit (no more) and pay it off each month. The result - assuming you're not still a f*kup - will be a surprisingly improved credit rating, and MUCH improved interest rates. The last person I know who did it that way raised his score from "extremely low" (he wouldn't admit what it was exactly) to 840. When he applied for an auto loan a couple of months ago, they offered to lend twice as much as he asked for (which of course he declined) and a "normal" interest rate.

Save some money, get a bicycle!

Co-signer is the best bet. Second best bet is to target new vehicles with high rebate + minimum 10% down at large volume dealerships. Banks are much more likely to process a loan on a new vehicle because of its warranty. High volume dealerships have more pull with banks because they can package loans together where 8-10 might be solid deals and they stick a couple of risky loans in with.

If you don't need a vehicle right now there are credit services that can help you raise your beacon and improve your credit. These services typically work by challenging erroneous negative credit. They'll also offer advice on how to quickly raise your credit score via secure credit.

The problem with going to a in-house financing situation is that you're not building your credit this way. While they might report if you're late, they will rarely report on-time payments.

Keep in mind that if you do go with a co-signer it has to be someone that makes sense. It can't just be joe blow off the street or a a good buddy. A spouse, a parent and sometimes a sibling can work, but unless the dealership has a shady finance department that's about it.

edit: Yeah as was mentioned earlier, Credit Unions are a very great place to start. Every town has that Credit Union that seems to approve loans that nobody else will touch. Become a member, keep a decent positive balance for a few months and then approach them. Also, stop letting people pull your credit. One of the quickest ways to not only destroy your beacon but also raise giant red flags with lenders is to have a ton of inquiries. It gives the impression that you're desperate and desperate people aren't reliable.