Korean-born foreigners qualify for a visa called F4. It is a two-year visa and is renewable without having to leave the country. Some benefits of this visa are that adoptees can work and tutor, own land, property or a business and you are not required to have a sponsor. To apply for this visa you will need various documents. It is possible to come to Korea on a 90 day tourist visa and then change to an F4, or you may try obtaining it while you are still in your home country.
The main Seoul Immigration office (#319-2, Shinjeong 6-dong, Yangcheon-gu, Seoul, telephone 02-650-6231 or 02-650-6234) is located at Omokyo station on subway line 5 (purple line) exit number 7. The office is about a 10 minute walk once you exit the subway system. Go to the second floor, room 204 and be sure to take a number upon arrival. When you apply for the F4 visa, your passport will be kept while your ID is being processed and you will get it back when you pick up your ID card.
You will need a copy of your hojeok (birth certificate) which you can get from your adoption agency. You may also need 2 certified copies of your hojeok which you can obtain from City Hall next to immigration :