As you make projects, your stash will naturally grow. I advise against going out and just randomly buying fabric on sale, unless you have a specific project in mind. The only exception I think is finding light print neutrals, black, white or other solids that you know you can use in future projects. When I first started out, I bought some 'stash' fabric, willy-nilly, many fat quarters and I still have a lot of it, as it just never was 'right' for a project.
Another important factor is your style of quilts, or the types of fabrics/designs you will make. I tend to make more modern, geometric pattern quilts, so florals, calico prints just hardly ever work for me, and some of the earlier fabrics I bought were these because I thought they were pretty. As you get more experience, you will learn your preferences. Now, I buy solid white and black by the bolt on sale, and most solid colors on sale too, as I know I will use them.