Well, it's good that you have an idea of what you want to do with your life. Most people twice your age don't. When I was your age, I wanted to be an auto mechanic, and I'm currently studying to become a UNIX/Linux Network Administrator....
Anywho, there aren't a lot of specific classes you need to take to get an A+. If all you want to do is general break/fix repair, than an A+ will be enough, but a MS cert for Windows XP and/or Vista definitely won't hurt any. To get your A+, the best way is to just start playing with computers, if you haven't already, which I'm sure you have
Also, pick up one of the many available study guides for the A+ exam. It's a 2 part exam, one on hardware and one on software (IE, Windows). If you can build a computer, and do some basic hardware trouble shooting, you're golden for the hardware. If you can clean out malware, install Windows, and do some basic software troubleshooting, than you're golden for the software part too. You can take the test and some courses for it in most High Schools, and at nearly any Community College/Universtiy.
I've done the Computer Technician thing... it's fun, but it's hard to do as a career. Computers are so disposable to so many people, that the call to fix them is either for
-OEMs like Dell, HP, Gateway
-GeekSquad, or another retail shop
-running your own business out of it. Usually doing residential/professional on-site repair.
See, a lot of businesses keep their computers under factory warranties, and unless you're factory certified, you can't even open the case without voiding that warranty. Few shops, if any will employ someone specifially to work on computers. If you'd be willing to get into networking and servers, it's another story entirely.
Aside from that, to really make a living, you have to start your own small business doing PC repair, and it's not easy. You need tools, you need really really competitive prices, you need clients, and then you have to listen to things like "You know the screws that hold the memory? Well, I think I broke the monitor." It gets old; really old, very very fast.
Hope I've helped some