You might want to check out the Asian board under the TER General Boards for some better advice, but, here goes....

Here are a few tips - first, if you can speak a little Korean it helps a LOT, if not, there is a part of Seoul called "Itaewon" that is an area that a lot foreigners live and work and you can get by fairly well just speaking English there.   There is an american-type bar and grill called "Geckos" a lot of girls hang there to meet guys.  But, if you don't want to have to fuck around trying to go that route, if you walk out of Geckos and walk directly across the street to the west, then down the hill about 100 feet there is an alley / street.  Walk up that street and on both sides are little places and the girls will come to the door and try and talk to you - those are hookers.  You will probably need to have Korean wan to pay them - they don't want to have to be trying to calculate the exchange rate for USD.

Good luck and have fun!!