I saw in Korean dramas that they don't pay using cash for public transportation?
Yes, Koreans usually use the transportation card: T-Money. Available in almost every convenience store, the card only costs 2,500 KRW. I highly suggest getting this card. You’ll get discounted for each ride (100 KRW). But even if you don’t use it 25 times to get the card’s worth back, the convenience and ease-of-use makes the investment more than worth it.