So my friends will arrive in Korea tomorrow and a day before,
they asked if they should take the taxi or bus. They wanted to take the
taxi because it will more much more simpler and easier for them but i
said NO because taxis are a rip off - especially the black one, they
would probably charge you 2x more because they can speak English. You
can simply just take the bus to Seoul for under $10. When i arrived, i
took a bus that stopped in front of Yonsei University. If you take a
taxi it will be over $60.

Also, if you want to make a domestic phone call at the airport, you
can purchase a T card, a rechargeable series of cards and other “smart”
devices used for paying transportation fares in and around Seoul and
other areas of South Korea. T-money can also be used in lieu of cash or
credit cards in some convenience stores and other businesses.

You can use it to:
Make a phone call
Make a purchase at selected stores
Pay for subway, taxi, and bus rides

Moreover, before you get here, it is best to learn some basic Korean
phrases such as how much, thank you, excuse me, and sorry. Some of the
natives here are intimidated by English so if you try to show them that
you tried to speak in Korean with them, they will be less nervous and
more willingly to help out.

Also, Korean food is VERY spicy so you may not get used to it the
first few days, but after a week, you will get used to it. So just in
case, i would advise you to bring over some medicine if your stomach
isn’t that good. In addition, American branded products here 2-3x more
expensive, so if you can’t live without having a certain item, i would
advise you to bring them over in bulk. EX: RED BULL IS NOT AVAILABLE