Your hamstrings are the main muscle group that allow you to run faster, so workouts targeting that group would be good. Also, flexible hamstrings and quads help in max performance. Sprints up a large hill works greatly.

You can also do ground level sprints in 10 yards burst, as previous writer wrote the main muscle group are hamstrings so
You should strengthen your legs with exercises like squats , step ups and lunges because these exercises use the same pathways that it takes to run with speed . Don't worry about leg extensions too much because the quadriceps will get plenty of work with other exercises. Instead, focus on strengthening the hamstrings with exercises like hamstring curls, straight-leg deadlifts and good mornings . The quadriceps shouldn't be significantly stronger than the hamstrings. If so, you are headed for injuries such as knee ACL tears . And, don't forget about your calf muscles. The calf muscles provide at least 30% of the strength and power it takes to run fast . Use calf raises (seated or standing) to strengthen the calves.

Once you have strengthened your legs adequately, you can progress to plyometric exercises to provide the power it takes to play sports with explosion.

I'd like to add one thing, weight training done correctly is a wonderful tool for young athletes. Done incorrectly, can be very bad. There is a big difference between training and "lifting". At a young age, the tendancy is to over-do the weight. Make sure you have some who knows how to train you before you hit the weights.
All the things that they said in the above posts are true, but you need to have a good knowledge of sets, repitions, correct form, and which exercises are the most beneficial for your age and physical ability.

Coach C

Just run a certain amount every day and maybe you can even time yourself.