Short answer: Get a job.
Medium answer: Get a job and don't be worrying about making money off your work yet.
Long answer: Get a job so you can pay for art supplies so you can focus on improving your craft because this early in the game, especially when you're 15, you're not going to be making a lot of money off your work . . . yet.
HOWEVER that's not to say you can't make money off your art at all. By the time I was graduated from high school, I already had my first children's book published (as the illustrator) and I was a serialized comic artist in a provincial magazine.
First off: Do what 69Erocento told you to do and throw out the "I'm selfish for wanting to make money off my work" mindset. It's your work, and when it comes to work, time is (or can be) money. I've seen people with the worst of the worst still get paid for commissions, but you have to go searching - you have to be in the small percentage of very popular artists to regularly get asked for commissions without even having to ask. Network yourself, update regularly on your art site/blog/whatever you use, keep looking for more opportunities to improve.
Second: Look out for leeches. When you're starting off it's going to seem enticing to take that commission offer where they don't pay but promise you things like "exposure". If it was that easy, we'd all be famous by now and we wouldn't have to be working second jobs at McDonald's, Starbucks, and office cubicles. Don't let yourself seem desperate, because people who will want to take advantage of you and your vulnerability will do so at the drop of a hat. Stick to your integrity here.
Third: Be very, very patient. If you do start making money off your art in some form or another, know that there are going to be highs, and there are going to be lows. There will be times you make $50 every week and there will be times you won't be able to find any offers or anything for months. Just like starting a new restaurant business or the like, you're going to have to wait for yourself to get out there.
But that's not the only place where patience is important. You're 15. There are going to be a lot of financial opportunities that you're not going to be able to take advantage of because you're a minor (ex. Ad Revenue on Tapastic to name one). It's frustrating, I know, but do know that there are a lot of other people being patient about this sort of thing too, so you're not alone. There are still lots of other things you can do.
Finally, most importantly, don't expect too much too soon. Don't get upset because you haven't made however much you thought you'd make after only a week of trying. Don't feel like because you didn't get famous overnight that it's a lost cause and you're never going to get to your goal. Be patient, but be assertive - don't settle for "I'm looking for commissions! I know my art sucks but....." sympathy draw-ins because I can tell you right now, people see through that BS in like 2 seconds and that doesn't make them suddenly want to care or give you their time/money. Hold yourself up high, walk with a firm step, and say, "This is my art! I've worked very hard to get to where I am and I am proud!"
And if you do end up getting a job in customer service or something to help pay for your supplies, don't look at it as a bad thing - it just means you now have more money to sustain your passion and you can put it towards your dream. There's nothing shameful in that in the slightest.
