Shampoo and comb!
A thorough shampoo with l% Permethrin
) is the most effective over-the-counter treatment.
This should kill all the live insects. Then it is very important to use a fine-tooth comb through the hair and remove all the tiny white eggs (nits). These may hatch later if not removed, and start the problem all over. There are also stronger prescription drugs that your dr. May recommend.