Get out there and meet people. That's all there is to it. Want to play at a particular venue? Get in touch with the owner or whoever does the booking and negotiate a gig. Want to play in a band with a musician you know? Talk to them. Want to play weddings? Get a booking agent (after you get a decent band together, first...) Need to put a decent band together? Meet musicians at their shows, scour craigslist, go through ALL of your contacts (somebody knows somebody who knows somebody...), put up flyers in your local music shop, etc...
Nobody gets handed anything, gig-wise, even in this era of "I can type ______ into Google and get immediate feedback." You've got to get out there and make it happen. The cool thing is, with all of the social-networking media and ways to get yourself seen/heard, it's easier than ever to make yourself known to others, but it's also easier for everyone else. Make your stand, make yourself stand out, and then get out there on the bandstand. The people that figure this out are the ones who get gigs. You're 15 and ready to gig--you've got a good head start. Go get 'em!