High 500s is pretty borderline, I'm afraid. Manny's advice is spot on - pay down your outstanding credit as much as possible since % of available credit is a big factor in your credit score. So is on-time payment history - if you have missed payments then the longer you go making on-time payments the more your credit score will go up. You're not going to lose very much just applying for a loan (unless you apply for a lot - and then creditors will want to know why you're applying for so much credit) but with a score in the upper 500s you're not likely to get a huge improvement over what you have now.
Why are you unlikely to get a much better interest rate right now? From a
blurb on credit scores:
Lenders began to take a closer look at FICO scores and this is what they found out. The chart below shows the likelihood of a ninety day delinquency for specific FICO scores.
FICO Score Odds of a delinquent account
595 2.25 to 1
600 4.5 to 1
615 9 to 1
630 18 to 1
645 36 to 1
660 72 to 1
680 144 to 1
700 288 to 1
780 576 to 1
Lenders don't really want to loan money at good rates to people who are likely to have problem paying their accounts on time. So start paying things down, try not to draw on your existing credit lines, make your payments on time, and in six months to a year your score should improve to the point that you'll be able to get a better rate.
Many of us have been where you are so don't beat yourself up - just resolve to do better going forward.
BTW I greatly disagree with the assertion that buying new is smarter. As soon as you walk out the door with the car you lose 15-30% of the value of the car. If you want a new car, that's fine, most of us do, but buying used is a better deal because most of the initial depreciation was paid by the guy who had the car before you. Right now small cars are retaining value much better but that simply means the dealer's not going to be motivated to give you a great deal on a new one because they can probably sell it at a premium to some guy tired of getting 8 MPG in his SUV.