There's 2 types of trains from the airport, express and all stop. The express train takes about 40+ minutes and the all stop takes close to 60+ minutes.

Express cost about 9,500 ~ 12,000 KRW (including transfer), while the all stop cost about 4,500 ~ 5,000 (including transfer to local subway).

Booking is online available for Express if I am not mistaken. For Express, you are guaranteed a seat.

It all depends on where you are heading (where your hotel / place you want to go). I took the all stop train though since I am OK with the slightly longer time and I can use the savings to go eat something

You can find out more info about the airport train here:
Airport Guide - VISITKOREA

Also, you are traveling with quite a number of luggage (or with kids or elderly with mobility issue), I would strongly suggest that you also consider taking a look at Airport Limousine Bus.

There's a lot of stairs in the subway ... and I do mean a lot ... there's elevator but it's often packed during peak hours and not all subway exit have an elevator !

Oh ya, get yourself Naver Maps, I used it while I was in Korea and it helped me a lot !