Your best bet is to practice doing sprints, the 100m shouldn't be too long a distance. You can start smaller and practice doing 50m sprints and then build up.
You should also make sure you get a decent amount of sleep the two nights before the event.
Because you will be doing sprints, during training and before the event I suggest you do a reasonable amount of warming up and stretches.
There is also another exercise where you sprint about 10 - 15 metres, touch the ground then sprint back another 10 - 15 metres and do several repetitions. So you are practising taking off from a point.