Preparation for Korea
If you wish to travel by bus, you can try several bus terminals in Seoul.
The "Express (Gangnam) Bus Terminal" (Subway Line 3 station called "Gangnam bus terminal") is the largest and the most popular terminal but doesn't necessarily have all the bus lines that go outside of Seoul.
Another big bus terminal is "Nambu Bus Terminal" (also a Subway Line 3 stop). This terminal offers buses going other directions out of Seoul than the Gangnam Bus Terminal. Make sure you check and go to the right terminal for your particular destination.
There are other smaller bus terminals in Seoul and in Incheon as well but usually for the most tourists places, these two big terminals mentioned above are where buses depart to and arrive from the most popular destinations. You can go directly to the venue to buy a ticket or you can make an online reservation.
Online Reservation :
There are two different options for which buses you can take. One is the standard bus (Regular one with standard seats) and the other one is the express bus (Slightly faster one with luxury comfort seats). It's recommended to book the express bus if your destination takes more than two hours.
Price range is from KRW 24.000 to KRW 28.000 for the standard bus (depending on destination) and for the express bus KRW 34.000 to KRW 38.000+ (depending on the destination).
Please note that the bus ticket price is very slowly increasing. Also, the buses will make a short stop on the way for the passengers so that they can use the toilet or buy some food or drinks on destinations that take longer than 2 hours.