If you like airplanes and helicopters, then you are making a wise choice. There is nothing worse than doing something you hate everyday. Trust me, I used to want to blow my brains out everyday working in an office. Now I am a helicopter mechanic for an EMS outfit and I love it. If you are going to an aircraft mechanic school, they will teach you the basics. The aviation community is helpful and always willing to teach new mechanics things.
As far as aviation mechanic schools go, look around before commiting to a school. You may be lucky and have a public school near you. There are actually high school programs that allow older people to attend and their tuition is much cheaper.
If you also are interested in computer stuff, you could always specialize in avionics. Another idea would be getting into the UAV/UAS field. Unmanned Aerial Vehicles are soon going to be operating in the United States airways thanks to a new bill passed by congress. This will open lost of opportunities for aircraft mechanics.
I would avoid the airlines as I have never met a happy airline aircraft mechanic. Cargo might be ok, not sure though. It will largely depend on where you want to live. EMS helicopter work is pretty good, I am salary and only work about 20-30 hours a week but on call a lot which doesn't bother me much since at least I am home with the fam.
Which ever you decide, put your heart into it and learn everything you can. It can be a tough market to break into but if you are really good at what you do, you can make good money,