I used to be able to sprint just under 12 seconds ish for 100m, though my speciality was 400m which i got down to a PB of 53.9 a few years back, since then ive lost a lot of my enthusiasm for athletics so I havent done it as much, but heres my 2 cents worth anyway
Sprinting is a very natural thing. If you can only sprint 100m in 15 seconds now, you never gonna get down below 12 seconds (assuming of course youve stopped growing, if you're like 13 years old or something then of course its different). Leg speed can be difficult to increase, however it is possible, and you can shave time off by building up more leg muscles which will aid you in acceleration a lot more. Eat a lot of carbohydrates when training, with some protein too (protein shakes if you really want to get into it, but dont get hooked on bodybuilding or anything).