When I first moved to Seoul, I had a hard time finding a place to live. There are so many 부동산 / real estate offices everywhere. Some years I would visit 10 different offices just to see about 20 places in the area I wanted to live. 부동산s share lots of information with each other, and share the commission. But it is important to note that often they keep the best apartments to themselves. For this reason, it is important to check around.
If you need to study Korean more and don't feel comfortable going to a 부동산 on your own, there are a couple sites you can check out. nearsubway.com is good for places all over Seoul, especially the Gangnam area. He is expensive but extremely friendly and helpful.
If you are looking around Itaewon, edenrealty.co.kr is a good site to check. Plenty of cheap places with low deposits. Call him up, his office is by Noksapyeong Station.
If you can read Korean well I would recommend a Naver cafe like Peter Pan. It would be easier to pay the same rents the Koreans pay using this site, and sometimes you can get a contract for less than a year.
Happy apartment hunting!