It is easy to be overwhelmed. Start small - Think of something small you can do (like sorting fabric and notions as already mentioned taking one box or stack). Set a goal of each day or each week to tackle one stack or bin. Organize anyway you want, but if you aren't a very organized person, start simple and just get a box for notions, a box for threads (or rulers or whatever you have a lot of), and a box or laundry basket for fabrics.
Fold your fabrics into the same size pieces (there are several tutorials on this board or look on youtube for "fabric stash folding" - there are videos. I didn't think I needed this until I realized all my different size folds meant I couldn't see what was underneath other fabrics.
Once you fold the fabric (its a really boring task, but essential), stack it up against a wall somewhere and let it sit until you are ready to separate by color or however you want.
Again, slow and easy - don't stress over the current mess - that gets you no where. Small steps and before you know it you will be more organized.