Check Your Credit
Before you worry about getting an auto loan with bad credit, find out if your credit score is actually low. Just because you've missed a few payments or have a lot of debt doesn't necessarily mean that your credit is bad, so check your credit and use those scores to decide your next steps.
Improve Your Score
Once you've checked your credit score, look for areas you can improve before you head to the dealership. While some aspects of your credit score can't be fixed quickly, errors on your report, old collection accounts, and other factors can be corrected to gain some points and get you a better auto loan rate when you decide to buy.