As your fifteen in your not already doing weights i recommend starting. Focus on big compound moves for upper and lower body like bench press squats pull ups deadlifts etc. this will develop strength therefore inverseley affect accelaration. thats why you see 100m sprinters with big muscles. i would also include some plyometric jumps and sled sprinting(10-20kg) for pure power/acceleration aswell it will also teach you good body angles hill sprints are and alternative to sled sprints but all sprints should be no more than 30m as you are not likely to sprint further than this in a game. you might also want to work on flexibility on your hips and legs as this will increase stride length and acceleration=stride length x stride speed
Search up the beep test. That is the best way. trust me
run stairs and uphills
What you really ought to do is talk to the sprinting coach for the track team at your high school. They will be able to offer the best advice as they can actually watch you run and assess your needs.