If a tooth is knocked out, the most important thing is to replace it as soon as possible . The tooth should be picked up by the crown (the part visible in the mouth normally) only. You should not touch the root.
The tooth should be rinsed briefly under cold water to dislodge any dirt. Do not scrub the tooth!
Still holding the crown, place the tooth gently back into the socket. (make sure it is the right way round - looking at the same tooth on the other side will help here.)
If the tooth can't be put back in the socket, the most important thing is for it to be stored properly until you get to the dentist. Milk is ideal, as it simulates conditions in the body quite well. If milk isn't available, water is better than nothing, but don't let the tooth dry out.
You must attend a dentist as quickly as possible. With all dental injuries, time is of critical importance, and will make the difference between possibly keeping the tooth, or surely losing it.