How can use the public transportation such as bus, metro, subway, taxi in Seoul, Korea?

After you arriving in Korea. And exchange your money as Korean money.
Then go to convenience store such as "ministop", "7-eleven", "GS25" or "CU". And buy the "T-money". You can also buy the "T-money" in metro, subway station or airport information desk.
"T-money" is a public transportation card which contains IC chip. After charging or recharging your "T-money", you can use it for transportation in everywhere in Korea. Is is highly cheaper than using your cash. Just remember, "T-money".