I am 16 years old and want to join the 2012olympics for track/field. how do i get into it and train properly?

Right now i can run 100 m in about 10 sec and i have had no training. i simply enjoy running. does anybody know how i can get into the summer olympics 2012? how would i get recognized? Is it expensive to be a sprinter in the olympics? how would i have to train myself? Thanks.