fitting can run from free (from a serotta certified fitter, no less, but that's because he's a former coach of my team) to $300. you fit is completely dependent upon what you want to do on the bike. as long as you're not racing (for which back parallel to the ground is preferred), the most important thing is just to keep your back flat.

here's something you could try. get on a trainer (or get someone to support your bike if you don't). make sure you have a mirror next to you so you can look at it to evaluate posture. get on that trainer and sit up straight. the goal here is that your torso will fall like the hand of a clock, going from 12'to about 10'. you should be contracting your abs while you let your torso fall. when your hands are on the handlebar, take note of your posture. is your back flat? if not, then you need to work on that. then, notice where your hands are when they make contact with the bar. ideally, the point of contact should be the top of your hoods. also, squeezing your shoulder blades is another good way to straighten your back and removes weight from your hands & transfer it to your core. the advantage of the latter may not be so obvious, but the thing is that you want as little weight on the handlebar as possible so that your bike will go straight without excessive input from your hands.

you'll find yourself getting into bad form for the first few rides, but if you actively think about it, your posture should improve in no time.