Make sure you are eligible for the pass.
The conditions are the following: If you are visiting Japan for temporary sightseeing (less than 90 days). If you are a Japanese national who lives permanently in a foreign country.
You should purchase an exchange order before entering Japan. Japanese travel agencies overseas sell the orders. Ask travel agencies in your area where you can get the order.
The price is fixed in Japanese yen, so your price depends on the yen rate on the day of purchase.
When you arrive in Japan, go to a Japan Rail Pass exchange office or a Travel Service Center located in major JR stations or airports.
Show your passport and the exchange order.
Tell them the day you want to start your travels.
Receive your pass with expiration date indicated.
You can use JR trains, JR ferries, and JR buses with your pass. Enjoy your trip!
clipped from: gojapan.about.com