Go to the express train ticket office and get a ticket to Seoul station. You will see signs for AREX after you leave customs on level B1. Follow the signs to the Customer Information Center on Level 1. It is orange with and I on it. Go to the ticket agent. It will cost 8,000 won. (about $8.00) You can pay by credit card.
There are several different ways to get from Incheon Airport to the downtown Seoul. The most convenient and affordable is the Airport Railroad Express (AREX)
If you traveled on Jeju Air, Asiana Airlines or Korea Air show the agent your ticket. You can receive a 20% discount.
There are two kinds of trains- the express train and the commuter rail. The express train will be carpeted and have blue seats and places for your luggage. Make sure to get on the correct train!
Seats are assigned on the Express train.
Look at your ticket. Your seat will be assigned by car and seat. Keep your ticket and plastic card you receive as you will need them to exit the train terminal at Seoul station.
The train takes 43 minutes.
For a step by step look on how to use the train and other public transportation to get from Incheon Airport to downtown Seoul visit: http://www.visitkorea.or.kr/ena/GK/GK_EN_2_2_2_3.jsp