How to repair my credit?

I was laid off twice this year, which really screwed me up in my bills, and now I have two debts in collection. I have about $12,000 in "good debt" (Car note, student loan) and 2,000 in bad debt (medical, credit cards). Do collection calls stop when you negotiate with your creditor? What is the fastest way to repair my credit? I use one pre-paid credit card and make a small purchase every month to pay in full. I don't make a lot of money (about 750.00 every two weeks), so I find myself living check to check. Thanks