Most subway stations are quite large, extending over a city block underground. They have several exits, and you have to know which one to take in order to get to your desired destination above ground.
Luckily, you will several handy guides. The "Way out" signs tell you where to go to find certain specific landmarks. They come in two forms, shown below. The red area at the top of the sign on the right shows a map of the station with the exit numbers marked.
Maps also show you the area around the subway station. The picture on the right is a closeup of the entire map shown at the left. While the names of the streets are in English and Korean, the names of the buildings are in Korean. Important: these maps are oriented in the direction that you are currently facing, so North is not always "up."
Riding the Subway
Subway