You can jump at the age of 16 in some places with parents signing off on it. At the age of 18 you can jump anywhere.
Very cool that you want to jump for charity. I just jump for fun.
There are a couple websites you need to check out to get good info on skydiving.
http://www.uspa.org is the website of the United States Parachute Association, the governing body for sport parachuting in the United States. There is a list of member dropzones on the website along with all sorts of great info on the sport.
http://www.dropzone.com is another great resource for everything about skydiving and some BASE too.
http://www.skydiveradio.com is just fun and a good place to feel like you’re at the drop zone when you can’t physically be there.
Make sure that you contact the drop zone you want to jump at directly. There are a few scams out there, one of which is 1800skyride. They will take your money and send you to any dropzone they want too but only about three dz’s in the country except skyride tickets. So please, please, please contact the dropzone directly.
The tandem is a great jump. I’d get video too. I made one tandem a little over two years ago and came back to start student training (good info about student training at uspa.org) and got my license. I’ve been jumping for two years and have a few hundred jumps. I love this sport and I sincerely hope that you’ll become a full time skydiver and I’ll be able to make some jumps with you at a boogie!
Blue Skies!