Before you can go about repairing your bad credit, or even trying to get a loan so you can rebuild your credit, you really need to know where you stand with your credit.
The only place you can receive a truly free credit report (without needing a credit card, or subscribing to a credit monitoring service) is at AnnualCreditReport.com. There you can get each of the 3 major credit reports for free each year. Consider getting one of the reports every 4 months or so, that way you can constantly keep up with your credit.
Knowing exactly what is on your credit report will help you see what items you can easily fix, and which items you'll need to figure out how to repair.