I easily get overwhelmed with where & how to start. What works for me is take one pile, bag or bin at a time. Put like things with like (notions, fabric, scraps, blocks quilt tops, books). If you have time - do another one.
Once everything is sorted and grouped then it's time to start making decisions as to how it will work best to organize and store each group. Where to put it, how to organize it and what to put it in? I find this to be the fun part. Again work on it in sections time & energy permitting. Fabric takes a long time to organize but having it done is a wonderful thing!
You can do it!