Start with interval style - run a sprint then allow for full recovery (let your heart rate and breathing return to normal) and then go again. As this becomes easier, start cutting down on the rest period. Also work with variable distances, from 20 yards all the way to 400 meters.
When you are in really good shape, you should be able to mix your interval sprinting in with various strength exercises. For example, run 400 meters then drop and do 30 pushups followed by 20 medicine ball slams. Then wait 30 seconds and do it all over again (keep going for multiple sets).