1. Incheon is the gateway to South Korea
Are you planning on going to the 2014 Incheon Asian Games by any chance? If yes, you won't have to make a big trip once arrive in South Korea. The de-facto airport where people arrive and leave Korea is called Incheon International Airport. Since 2005, ACI (Airports Council International) named the Incheon airport as "the best airport in the world" every single year. Not bad, considering the airport opened in 2001.
2. Won (₩) is the Korean currency of money
Once you arrive at the airport, make sure you exchange your money to the Korean won. One dollar (USD) is worth about 1,000 won. You can convert other currencies to the Korean won here. Look at the list below and get an idea of how much things cost in South Korea.
5,000 won (₩5,000) = a cup of coffee
10,000 won (₩10,000) = a typical Korean meal
50,000 won (₩50,000) = a cell phone bill + an Internet bill
100,000 won (₩100,000) = a very expensive hair cut!
3. Know how to get around once you are in Korea
Know how to use the subway system and how to ride a taxi. There are public buses as well, but they can be overwhelming for first-timers. Get an online subway map here in English, Korean, Chinese, and Japanese. Incheon Line 1 is connected to the Seoul lines on the far left side of the map. Taxi fares are pretty cheap, but don't expect taxi drivers to speak much English. Once you are inside a taxi, simply say, "(the location) + yoh" and the driver will get you there. Hopefully.
4. Know the climate (before you pack)
Nothing is more frustrating than finding out later that you packed wrong kinds of clothes. Korea is really hot during summer and really cold during winter; usually ranging from 0 to 32 °C (32 to 90 °F). It can even get colder or hotter than that, so make sure you bring proper attire for the right season. December and January are the coldest months and July and August are the hottest. It also rains a lot (and I mean A LOT) during summer in Seoul. So having an umbrella is a must.
5. Say no to drugs
South Korea is very tough on drugs, very tough. Consider Korea as a drug-free country with no exceptions.
Rule #1: Do not possess any drug with you at all times.
Rule #2: Do not talk about doing drugs to anyone.
Rule #3: Do not do drugs while you are in South Korea.
If the police find out that you are doing drugs, you may get a free tour to a Korean prison. That may not be a risk you are willing to take.
6. Know some Korean language
Here's a list of how to say simple things in Korean. These short phrases can really be helpful anytime you interact with Korean speakers.
Hi / hello = ahn-nyuhng-hah-seh-yoh
Hello (during a phone call) = yuh-boh-seh-yoh
Goodbye = ahn-nyuhng-hee-gah-seh-yoh
Thank you = goh-mahb-seub-nee-dah / gahm-sah-hahb-nee-dah
I'm sorry = jweh-sohng-hae-yoh
How much is this? = (ee-guh) uhl-mah-yeah-yoh?
I want to go here = yuh-gee gah-goh-sheep-uh-yoh
OK / good = joh-wah-yoh
Yes = neh / yeah
No = ah-nee-yoh
7. Bring deodorant
You can find almost anything in South Korea. Starbucks, McDonald's, Apple stores, etc. But it may not be so easy to find personal hygiene stuff like deodorant in South Korea. So make sure that you bring some with you and bring plenty. You can always give the remainders to people around you as gifts. (Well, maybe not the used ones...)
8. Know the culture
In South Korea, tipping is not part of Korean culture. You don't have to tip when you go to a restaurant or a hotel. There's no tipping for taxi rides either.
When you say hello, goodbye, thank you, or I'm sorry to someone older than you, bow your head a little. Do not talk too loudly in public. (Foreign language sounds are perceived as louder for some reason.)
When you give or hand something to another person, use two hands instead of one especially to an older person.
All types of religions are tolerated and respected in South Korea. People do not carry guns and streets are pretty much safe. Many Koreans speak a little bit of English but do not expect them to speak fluently.
9. Remember this website!
That's right. 9korea.com. (We have no shame.) Here is a list of how you can use this site to your advantage while you are staying in Korea.
Dictionary - learn Koreans words on the fly (Use the random word feature)
Subway - get subway maps in different languages
Weather - real-time weather and forecast information of South Korea in English
News - get the latest, most important Korea news in English
World Clock - find the current time in major cities around the world
Calendar - find out important dates and holidays of South Korea
Easy Korean - learn how to speak Korean with free YouTube videos
Map - online maps in English and Korean
Unit Converter - convert one unit to another
Currency Converter - convert different currencies to the Korean won and vice versa