If it is a permanent tooth, rinse it gently in cool water. Do not scrub! Do not clean it with soap!
Put the tooth back into the socket immediately and hold it there with a clean wash cloth or gauze. If it is not possible to put the tooth back, then place the tooth in a cup of cold milk or water. See a dentist A.S.A.P. (30 minutes or less will ensure a better chance of saving the tooth).
If it is a baby tooth, do not put it back into the socket! It can cause damage to the developing permanent tooth. Go see a dentist soon.