Find the tooth, and handle it by the smooth top (crown). You may rinse the tooth but DO NOT clean the root portion or handle unnecessarily.
Attempt to reinsert the tooth into its socket. If the root is not dirty or damaged from mishandling, this is your best opportunity for successful reattachment, so just do it! Hold the tooth in place by biting on a clean gauze or cloth.
Contact a dentist immediately. If reinsertion is not possible, carry the tooth in milk or place in water if milk in unavailable. Time is a critical factor in saving the tooth by replacement.