My 3 is also my first car, and a great one at that. You want something that is safe, reliable, and has enough power to get out of its own way, but not enough to get you in trouble. Good looks don't hurt
As for the 2.0 vs 2.3, go with whichever you can afford. Both are very good engines, and if you're starting out, you probably don't want or need the extra power while you are still getting used to driving on the street.
Also, I would recommend getting a manual transmission. I learned how to drive and learned how to drive stick at the same time; it made me a much better driver because the MT forces you to focus on the road and on your car rather than fiddling with the stereo or your cell phone. Also, you will become much more intimate with your car. The first three months are hard, though, but as you pull out of them, you realize what a great choice you made. Plus, I bet none of your friends will be able to drive your car.
Ninja edit: PAY FOR SAFETY! Make sure the 3 you get has the optional airbags/ABS, and if you manage to miraculously get an 07 model or later, you could snag traction control and dynamic stability control.