Use a product made especially for killing lice and nits.
Several products are available without a prescription and work well.

They are easy to find at stores which carry hair care products.

They are also available at the drugstore. They come with specific instructions for use. It's important to follow the directions on the product EXACTLY. Do not over-use these products. The shampoo treatment may reduce the number of lice by 30 - 70% but they will have little, if any, effect on the eggs (nits). After treatment, you will need to use a special metal comb to comb out the hair. You can purchase these combs separately, but most of the time they will come in the box with the shampoo. You can use the special metal comb on other members of the family if they get a case of head lice and need to be treated. However, the special metal comb must be cleaned properly first!!!!!!!!!!