Don't take out a loan to pay for a downpayment. In fact, never take out a loan to pay for part of another loan. That'll only hurt your credit score even more as there will be more debt held against you. Keep one credit card (each additional dings your credit) and simply pay off the balance every month. You don't need to do interest payments.
Keep a job, stay in school, avoid debt and new forms of credit (like credit cards). Your credit score will build over time.
I also wouldn't have your parents on the hook for co-signing. Co-signing is an incredible evil that can wreck the co-signer's finances and credit if something goes wrong. There are times when it is needed, but buying a new car with little credit is not one of them.
As for new cars...
I don't think I'll ever buy a new car unless there are some special subsidies or deal that allow me to get it for a huge percentage off. Both my cars (one bought after I lost the first one in a crash) I bought on Craigslist and both worked perfectly. My current car, a Subaru Outback '05 was the best model Subaru made in that year and I got it in excellent condition with about 90k miles on it. For the same price I paid to buy it outright, which was around $10k, the best I could've I could have only gotten new was some lowest-end tiny econocar or some crap like that with no features, if even that. I could've gone quite a bit lower in price too if I didn't want a particular engine in that model.
One thing some of the newer cars have going for them is better gas mileage, but you don't need the newest model to get that kind of mileage. Plus, you might as well wait until good gas mileage is actually needed. Right now gas is cheap, so enjoy it while you can.
:beer:
