I know that cardio will increase my endurance and such, but how do I improve my sprinting like in the 100m? I play lacrosse and I want to improve my speed, how could I do this?
A good way to increase your speed is to do downhill sprints. These help leg speed considerably, as long as you don't trip. If you do, hang on cause you're going for a looong slide.
Uphill sprints or running up stairs also help with your explosive power to get you out of the blocks.
Plyometrics is really the only way to tap all the potential of your current strength.
I'm a basketball player that ran track to keep in shape, but my 100m time was in the 12.2's, and my 400m time was in the 52.8's.