Keep buying as many tunes as you can afford, and every month or so do a fresh demo on CD. Make a personalised cover you can re-print easily, can be very basic.
Doesnt have to be a long one, just to represent your sound / mixing style. Promoters get loads of demos and if you havent got them hooked after half an hour forget it.
Make sure your name and contact details are all neat and visible on both cover and CD.
Get your face known, one of the hardest things for people who arent outgoing. Find out who promoters are in your hood and approach them straight up, don't blag or brag, just tell them you are ready to play and hand over demo.
I dont know if over-18 club promoters will give a gig to a 15 yo but it worked for JMajik and various others so HEY go for it mate.
Never, ever stop practicing. Approach every promoter you can, even ones you might not go to their parties. Utilise the internet to the max if you can, but don't rely on it.
Big up yourself mate, good luck to you too.