Citizens of several countries, including the United States, Canada, Australia, New Zealand, and many European countries, can enter Japan for short-term stays (usually up to 90 days) without a visa for purposes such as tourism.
However, visa requirements can change, so it's always a good idea to check the most current information from an official source or the Japanese embassy or consulate in your country.
You must have a valid passport and an onward/return ticket, and your passport must be valid for the entire time you are staying in Japan. Don't be surprised if an immigration officer asks you to show proof of sufficient funds to travel for the length of your stay in Japan.