Please look at the list of the countries that have the visa exemption agreement with Korea. If your country is included in the list, you usually do not need to apply for an entry visa to Korea.
If your country is not included in the list, you need to obtain a short-term stay visa from the embassy of Korea in your country.
Read carefully the Korea visa guide provided by the ministry of foreign affairs. Please be informed that it is the embassy of Korea 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 soon as possible.