I would suggest finding a pattern for a quilt that you want to make. The instructions will tell you how many colors and how much you need of each one. You will generally have a light, a dark and some medium tones. You pick those fabrics out of your stash that go well together.
I, too, bought a lot of fabric as soon as I started quilting. (Luckily, I didn't pay a lot for it, as most came from estate sales.) Still, when I found something I wanted to make, I didn't have fabric that worked in my collection. I've donated a lot of it.
If you want to make scrappy quilts, you can use most anything. Still you need contrast.
Most of my quilts have been I spy quilts, so I can use anything for the back, cornerstones or sashing (within reason).
So, when I get the urge to buy fabric and I don't have a pattern in mind, I buy something that will work for an I Spy quilt.
bkay