Hello and welcome from an Iowa girl!
So you want to build a stash? First and foremost buy quality fabric. Discover what kind of sewing/quilting you will be doing--large quilts vs table runners or both. Discover the style of fabric you want to work with--Modern quilt movement or traditional or somewhere in between.
Buying on sale is good, but only if you really like it and it will work with fabrics you already have.
I'm sure others will provide more ideas. Best of luck in pursuing this wonderful hobby.