in no particular order:
contact your local athetics club / join them
try speedwork drills (10 x 200m, 10 x 400m, 6 x 800m, 3 x 1600m, all with equivalent rests etc)
do a couple of slow / steady runs each week too
Bearing in mind your age, and assuming you have not yet stopped growing, I would be cautious about increasing mileage too much, hence getting advice from a local club that works with "children"
Stick around the forum threads on RW as there are thousands of people willing to help!
Good luck!