What you should be doing
You're going to have a huge problem getting approved for anything as long as you have an unpaid bill on your report. Pay it and make sure its reported as paid in full - ASAP.
Once that settled, your credit will start to improve slowly. Can't do anything about that, it will take time.
You can make the situation improve a bit faster by lending money to yourself and having it reported regularly on your report. How? Easy.
Get a secured credit card. What does it mean? You put X amount of money in a CD and the bank will issue you a credit card secured by that CD. Your credit line will be based on the amount in that CD, and you'll probably pay some fees to the bank for the service (~$20-50/year, shop around). You might get lucky and find a secured card without fees, if you look hard enough.
Secured cards are reported as revolving credit (just as any other credit card) and are easy to get because the bank doesn't take the risk - you do.
If you default on your payments - your CD goes to cover the debt, and the card gets cancelled cancelled. But make absolutely sure that you do not default. Charge between 10% and 30% of the credit limit each month, not more. Pay the balance shown on your credit card statement in full every month and by the due date shown on your monthly statement.
It will take a while, but you would typically start noticing the improvement within ~6-12 months.
What you shouldn't be doing
Stop applying for stuff. Not store cards, not car loans, you're not going to get anything, and will just keep dragging your scores down. Each time you have a pull on your report, the score goes down. A lot of pulls, frequent pulls - the score goes down a lot. Lenders can see when one is desperate, and no-one wants to lend money to desperate people.
Optimally lenders want to lend money to people who doesn't need loans, but in order to keep the business running they'll settle for slightly less - people who don't usually need loans, and pay the loans they do have on time. You fail on both, as you're desperate for a loan and you have unpaid bills on your report.