If you suspect that your child has been exposed to lice, visit a doctor immediately and request that he prescribe a lice-killing lotion or shampoo. Medicated shampoos are often able to kill the lice soon after use. The best way to prevent getting lice after being exposed to them is to stop them in their tracks so they don't reproduce and become a bigger problem.
If you've been given lice shampoo, follow the directions precisely. Because the shampoos include insecticides, using too much shampoo or using it too often can be dangerous. Also, If the shampoo is prescribed for two weeks, use it for two weeks---even if it seems like there are no lice after a few days of treatment.