As mentioned above, the best way to choose a bike is to take your time in the store fiddling with it, ask millions of questions, test it out, and then trust your own instinct.

As for using commuting for getting into shape, it's pretty astonishing how biking will help over, in my opinion, other methods like running, etc. If you like biking you'll stay at it longer and I personally found that even easy biking at 10 mph without challenging hills and taking the time to enjoy the process has dropped another ten pounds that strict dieting and walking could not get eradicate.

Please though, as a newbie to commuting biking, pace yourself, don't overdo it and you will find rewards on many levels. I bike/bus and only have to ride about ten minutes to get to the bus stop and all on flat land. As I get stronger, I bike to the next bus stop and then the next and never to the point that exhausts me.

Figure out what your philosophy is about biking. In my case, I will never enter a race or go very long distances, or constantly face major hills. I'm in it for comfort, fun, the challenge of gradually stretching my biking boundaries all heaped onto the initial impetus of high gas prices and wanting to simplify my life away from the death-grip that cars have on Americans. And beneath all of that is the burned in memory of myself as a child learning to ride a bike and that ultimate transcendental sense of freedom and accomplishment and peace when I could finally balance it without falling and ride with the wind.

Good luck and keep in touch.