First make sure that the information shown on the reports from all three bureaus is correct. The site to get all three for FREE is:
www.annualcreditreport.com/cra/index.jsp . If you want your scores, there is a charge. Often when you pay off a collection or charge off, the creditor fails to report this. Such errors can be disputed. If you have no good recent credit to offset the bad, you might consider getting a "secured" credit card. Try not to use more than a third of the credit limit and pay the balance off every month. That way you won't be paying the high rate they charge. After a reasonable time, you should begin to get offers for unsecured cards. At that point you can request your security deposit back. If they refuse, you can cancel that account and open a different one. Your efforts to fix your credit will be well rewarded.