If you can read Korean then you might want to check out this site:
Help with moving to Korea can be found here. The article offers advice on how to find accomodation before leaving and what to do if you can't find accomodation in time. Don't panic, just read.
Expat Evan also has advice on how to rent an apartment in Seoul for first time renters. His advice is offered in English, and is a must-read for those not sure even where to start.