You may not want to hear this, but there's really not much point (I would personally say there's none) in trying to drive a racing simulator without a wheel and pedals. That's the very first point. Of course, that assumes that you're genuinely interested in learning how to drive at high speeds; if you're not really that interested, then all bets are off. I think that if you are interested, you could do yourself a favor and buy the DVD Going Faster by the Skip Barber school, and watch it enough times to absorb the lessons. Then practice the techniques in your simulator.
It's lots of laps, trying to lower your lap times, that will let you advance. Changing setups is advanced stuff; there's no need to experiment with that till you're very accomplished with the default setups.
I'd say that it's useful to drive a variety of cars to begin with, and choose which one you happen to like most, then drive the heck out of that one. You'll learn the car before you learn a track well, and way before you learn all the nuances of a track. As you try to lower your lap times, first you'll be looking for seconds; then you'll be looking for tenths. Not until that point would there be any purpose to changing setups, and then not random experimentation, but based on tutorials written by knowledgeable people. I'm pretty sure there's at least one such tutorial right here on RD.
