I had the same issue with my ADD teen son. We took a weekend day and broke it all down. First I went and purchased some organizing tools such as baskets, trash can, containers. We went through everything and he decided keep or toss. It was hard for me to keep some of the things that he wanted to hold on to, but I kept my mouth shut and let him decide. Then we sorted through and put like things together, with him deciding where it should go. In the process we rearranged his room as well; so it was like a brand new start. It took all day, but the result was beautiful and he felt proud afterward. Then we talked about how to keep it this way. So, now I give him reminders on when something needs to be picked up or organized. This new start has helped him to keep the clean up small and manageable. Its been kept neat and tidy for about 5 months now. 🙂 Good luck!