Credit repair companies are legal. They don't do anything you can't do on your own. You're just paying for them to do the leg work. It's essentially one big technicality. These companies ask for verification of the debt, late payment, etc. Apparently a lot of companies don't do a great job of keeping that documentation. If they can't provide it, it must be removed from your credit report. Again, nothing you can't do on your own. If you have outstanding debt on your credit report, getting them removed either through dispute or by paying them off will make your score jump pretty quickly.
Moving forward, the most important thing you can do is make sure every payment is made on time. Keeping your utilization rate under 30% is good, 15% is ideal. if you can pay them off in full, even better. The key is to learn when your statement cycle closes (you can find this on your monthly statement). This is not the same date as when your payment is due. Whatever your balance is when the statement cycle closes is what will be reported to the credit bureaus.
Finally, you will need some active, positive trade lines to help increase your score. You can do this by opening 2-3 secured credit cards. The credit limit doesn't matter ($300 is fine). Again, make sure your payment is on time and your balance is below 15% of the credit limit when your statement cycle closes.