go to "deviantarts" that is the best website for arts.me and my big brother are already members there...my big brother is obsessed with that website..you should check that website and begin your career there........
At 15, art class would be a good start. Then practice practice practice....and study study study...
you could go to some cafes and things and ask if they will display your paintings
buying and seeling every thin is there
i am also member there