It varies with each project for me. I prefer to let a fabric find me, then pull additional fabrics to work with the original fabric. I pull all kinds of values and colors that will play well together. Then...I mull them all around in my head for several days. Usually I find that the fabrics seem to call for a specific type of design..traditional type block, more abstract, stars, etc. Then if I can't find a pattern that fits what is in my head in one of the books I already own, I make up my own pattern (its way more fun that way).
I guess there are as many ways to do this as there are quilters.