I'm also interested in makeup as a career for myself.
Highschools - not much there, but you can do makeup for plays and other events, that should count as something. Also, try talking to a guidance counselor for advice, I do that and I've found so much!
If you know any friends who are good at makeup, you could try learning from them, or you could search the internet for tips. MAC allows you to chat with their artists online or email them, so maybe that could also help.
You'd probably have to go around offering to do people's makeup for free for a while to get a portfolio and build up a list of clients (freelancing). A portfolio should also include a short paper to accompany the pictures (inspirations, about you etc). And make sure that everything in your portfolio is professional - no badly torn out pages, typos, ANYTHING! Eventually you could start prices though, and that's when you should be considered a makeup artist and able to qualify for a pro card. I don't believe that MAC requires you to go to a cosmetology school for the card - only if makeup is part of your main career.
Buy some makeup books? There's a
FabGuide to being a makeup artist, and many books out there, but make sure you read reviews first! Some books tell techniques where others can just be pictures.
It would also help to know much of the fashion world (trends, etc). It shows your intelligence and also can help you to put the right thing on the right people.
I hope these things help - after all, I'm only 15!