How can I get my credit "good" again?

When I turned 18, I had excellent credit, since I had been paying on my own car loans and cell phones with my stepmom as a cosigner. I was on my own at 18, working two jobs, going to school. It was all a lot, and after awhile I was unable to make my credit card payments, that I had charged a lot of debt on to survive. Because I was so young, I couldn't even afford insurance on my cars. I tried every other month or so to get insurance, but kept falling short and being dropped off insurance. Anyway, I haven't had a credit card since 2004 and haven't made payments since then either. The same with a cell phone in my name. I think the insurance claims are from 2005. Anyway, I pulled my credit report last winter and it was in the 400s. I had claims, like the insurance, that I didn't even know were on my report. I'm now 24. I would like to be able to buy a better car and maybe have a credit card for emergencies, since now I can afford the payments. Anyway, after my long story, how can I fix this problem I've created? I can't pay off all the debt, and I don't know if I even should, since they can't collect most of the money any more. Can the negative information be removed soon, since it's approaching 7 years? I made some mistakes, but I'm not a child anymore. Are my past credit mistakes going to ruin my life forever? What are the best steps I can take now?