I'm new too! Please wait for the replies of more seasoned members, but I would start with the free sites like credit karma. They are NOT accurate but they are a good start. First checking to see if you have anything not valid, and then disputing those (disputing yourself the online dispute isn't that great), then checking collections you want to try to do payment for deletion on anything you can. I would say after a little while you could purchase all your scores, I would say a site that gives you a monthly score is best and acess to all three reports, like the one on this site.
The first step is always knowing exactly what you have. I used credit karma and looked up the number to a medical collection company, I told them I never knew about the debt and they wrote me back today saying they would delete it for payment. 
Good luck.
After you look over your reports you'll need to put together a goodwill plan of attack regarding all those late payments.
If those numbers are accurate you have a lot of work to do.
Late payments being reported on your credit reports most certainly do not help you.
They are the reason your scores are in the 500's. Goodwill letters to the creditors asking for removal of the lates/removal of the tradelines can remedy the low scores... otherwise they are on your report for 7 years.
If I were a lender and saw someone with a dozen lates on multiple accounts I wouldn't loan them enough money to buy a hamburger off the dollar menu... just the way it is.