I start with how much I can afford to spend. Then I look to see what strikes me at that moment. If I have only one that I want I spend as much as I have on me at that time. If I want several then I make a choice as to which ones I can't live without. Sometimes I talk myself out of something that isn't as good as the one or two or so that I just have !!! to have. Something I have been looking for and couldn't find or something that fits exactly what I need etc.
I store my fabric on shelves by color. I have floor to ceiling shelves behind my sewing machine. Yes it needs to be refolded and reorganized by value but I can still see what i have and overlook that it didn't get put back where it belongs! Oh well!
I get fabric at yard sales buy clothes at good will for the fabric and look for large dresses and skirts especially as they are good fabric if they are cotton.
You will find what works for you!