I just wanted to say Hi, and that it is very refreshing to see someone actually read the newbie section before posting. That way you can ask intelligent questions. Your definately on a good start.
Now to try to answer some of your questions. Im not sure if this is true, Im just stating this as I understand it. Jason or a Mod can correct me if Im wrong. You can attend an AA game if you have your parent attending with you either playing in the field with you (and you must stay with in so many feet of your guardian) or your guardian must be in the staging area at all times. Plus a wavier will need to be filled out. The sponsorship is meant to allow the minor to be able to play without having a parent or legal guardian at the event, since an arrangement has been worked out with the sponsor and the sponsorees parents. Basically determining that the sponsor is responsible for the minor, I think some legal forms may need to be filled out that may restrict liability on part of the sponsor. This is how I have come to understand it for the most part.
For mags, most guys use standards. Others use hi-caps and drum mags. Go for what you will want, More realism or an infinite ammo supply. Some also use standards and have one hi-cap for emergencies.
For guns, a TM is a good gun. Classic Army builds an M4 that has more metal on it and better internals than the TM. Thus when you want to upgrade the fps of your gun you dont have to invest to much more money into it. One thing that I highly recommend for anyone who gets a new CA is to take off that crappy white gear grease and put some better grease on it. (can't remember the kind of grease, Scott hooked me up here and its just night and day.) The grease CA uses is not enough in the first place and secondly it hardens up after awhile.
Good luck to you and your new hobby in airsoft. As others have said before me, If you keep a mature attitude you will do well here.