I've been to Korea with some friends two years ago and we could simply buy the tickets at the station. Most stations will understand English, and the ones you are going to should. Smaller/less touristic ones like the station in Mokpo will have some issues with English but nothing too big to not be able to buy tickets.
Be aware that:
- Normally you will get backwards facing seats
- Forward facing seats will cost (slightly) more
- Family seats (2 sets of 2 seats facing each other will also cost (slightly) more
Also, if you are going from Seoul directly to Busan or from Busan directly to Seoul do try to get the "direct" KTX. That will take about 1 hour of your travel time compared to one that stops at more stations.
Lastly, when you are leaving Seoul please do make sure that you have at least enough cash with you to get a return ticket and maybe some food. We made the mistake of not doing that to a couple of the smaller cities where the KTX stopped and had to walk back to the station get to an international ATM.