You've gotten great advice here (as always!). I first pick out the pattern I want to use then move onto the fabric selection. As the others have said, I pick out the main focus fabric - or at least main color I want - first. Sometimes I'll pick out a couple of different main fabrics as it might be easier to get the remaining colors to work with one of those focus fabrics than another. When I get the first couple of colors together, I'll lay the bolts next to or on top of another with proportionate amounts of the fabric on the bolt showing that will be in the quilt. Kind of like the 'squinty look' that someone else mentioned. Any one or more of the colors or just the fabrics themselves might change a few times before I have it all worked out. I don't go fabric shopping when I have somewhere to be at a certain time!
Many times the sales folks, particularly in a LQS, will be more than happy to help you with fabric/color selection. Sometimes I'll have it all picked out, look at it for a bit and completely change my mind, put it all back and try again another day.
As to stash...I'll probably be one of the few people here to tell you not to worry about it. Particularly at this stage in your quilting career. Your tastes will likely change ALOT in the coming years and what you think is really great right now could easily become one of those 'what was I thinking?' fabrics. Just overbuy for your projects. That way you will have enough fabric to account for booboos in your current project and you'll end up with a bit of a stash over time. I don't have room for a stash and don't have to worry about the bucks I'm spending on it either. Or deciding that after a couple of years I'll use x fabric for this quilt only to not have enough of it for where in the quilt I want to use it and then can't find more of it.
Just relax and enjoy the process. It's not a race.