If you have the piece of tooth and you want it re-attached (assuming it is not a small piece and your roots are exposed), as soon as possible store it in saliva, milk or salt water. If you are going to the dentist no later than a day, you can put the piece of tooth back on your tooth, and wrap a piece of foil over it to keep it in place. Obviousely you will have to be careful when you eat, but if you press it over your teeth and gums securely it should work, or use a temporary dental cement.
If you havent made an appointment yet or it is days away store the tooth like i said. Also put a cold compress over your lip or cheek where the tooth is to reduce any swelling if there is any.
You can use over the counter pain relievers like ibuprofen (advil), rinse your mouth with warm water. If there is any bleeding apply gauze for 10 minutes.
I hope this helps.