Please look at the list of the countries that have the visa exemption agreement with Japan. If your country is included in the list, you will not be required for a visa when entering Japan under normal circumstances.
If your country is not included in the list, you need to obtain a short-term stay visa (“tanki-shouyou visa” in Japanese) from the embassy of Japan in your country.
Read the Japanese visa guide provided by the ministry of foreign affairs carefully. Please be informed that it is the embassy of Japan in your country that makes a decision on issuing a visa. Usually the procedure takes considerably long, so you need to begin the procedure as early as possible.