If your credit score is absolutely dismal you're right to worry. Fortunately this is not the end of the road and there are actions you can take to improve your credit score, even if it is as low as the 400s. There are people that have come back from bankruptcy to fix their credit and now have stellar scores. Although something like that could take up to 10 years it is definitely worth the effort.
The first step would be to order your credit report. Make sure you get one from all three credit reporting agencies as well. It's important that you get all three reports to get a full picture. Lenders are not required to send information to any of the bureaus and most often they do not send information to all three. That is why sometimes you will have negative marks on one report and not the others.
Once you have your reports in front of you go over every section very thoroughly. Check to make sure that there are no mistakes, for example a note indicating there was a late payment when in fact all of your payments had been prompt and on time. Like yourself the people that work at the credit bureaus are human and make mistakes. If you do not catch these mistakes trust me, nobody else will. Should you find any mistakes all the have to do is notify the Bureau and the lender involved both by phone and in writing. The Bureau will then have 30 days from receiving your note to investigate the issue. In your letter I suggest you mention that you will be following up with them from time to time. Remember just because an issue is investigated this not mean any action to remove it from your credit report will be taken. That is why you must follow up and make sure that everything is taken care of to your standards.
The next thing you must do is list your link when it counts and create a plan to start repaying them as soon as possible. I would suggest listing your accounts by the balance owed and start paying off the lowest balances first. This way you can start knocking off those extra interest rate payments as soon as possible. Another option would be to start paying off the highest interest loans first.
Creditors use your payment history as a giant chunk of the formula for calculating your credit score. That is why it will take some time before your score starts to rise simply from paying your bills on time. After you have shown a steady history of on-time payments is when you'll see the most improvement in your score.