Originally Posted by
TejanoTrackie
The only really bad habit you might get is getting a bike geared too high and not learn to spin rather than mash when you pedal. Also, running a big gear at your age when you are still growing can be bad for your joints. So if you do choose to buy something like a bikesdirect bike, get a larger rear cog like an 18T or 19T, which will give you gearing around 70 gear inches or less. Make sure you have good foot retention and use at least a front brake.
+1 to this. Don't overgear - even 70 inches might be quite high for a complete newb who is still growing, and it's dirt cheap to change the cog for a higher gear as you get stronger and more used to it. And buy a bike that can flip from fixed to singlespeed, lke WoundedKnee suggested. if you're riding with friends on geared bikes, however good you get at spinning you'll find it impossible to keep up with them on significant downhills on fixed so you're sometimes going to find it useful to be able to coast. You'll blow them away on the uphills, though - after a while.