1) Learn to Read Hangeul
This is the #1 thing I wished I'd done prior to coming to Korea. While there is some English here and there, I'd say 85-90% of what you see will be in Hangeul. "But Nick," you say. "I won't understand the language even if I can read the letters. What does it matter?" To this, I have two points. First, you'd be surprised how much English there is that's just written out in Hangeul (Korean and English do share a handful of words). Second, it's mostly helpful when you're trying to find something, whether it's a place or an item.
Fortunately, Hangeul is amazingly simple to learn. If you take a few hours and sit down to practice, you can learn the alphabet in a day (or less). And with practice, you can read it a bit faster. Here's a fantastic website/comic strip that makes it even easier:
2) Prepare Yourself for the First Month
Your first month is going to be rough. It will take that long to get your ARC (Alien Registration Card), which you'll need before you can get pretty much anything. You need it to set up your bank account. And you need your bank account to do stuff like set up a phone contract/get internet/TV/etc. So unless you come with an international plan on your phone, you'll be stuck in the dark ages for a while, and this affects more than you realize. Fortunately, I came through KorVia, who provided me with a temporary phone to use as soon as I got here. It's no smart phone, though, so it's calls and texts only. Still, it's pretty helpful.
To prepare yourself, make sure you have stuff to do in your downtime. If you like video games, make sure to bring some with you. If you're a reader or a film fan, have a Kindle or some movies to keep you occupied for a month. Basically, just have something to do while you're at home and not out exploring things. There are also Internet Cafes you can find pretty easily. They're cheap, and you can check your email, etc., from there.
Also, during your first month, you will most likely get sick. You're in a different part of the world with different allergens and a different climate. It will take some time for your body to adjust. So make sure you bring some basic medicine with you to get you through (later you can figure out how to get medicine here, but it's not as simple as in, say, the States where you can just go into a store and pick something off a shelf).
3) Korean Culture is not Your Culture
Seems pretty basic, but you'd be surprised how many people come here expecting Koreans to not only act the same as you, but accept how you behave. While not drastically different from western cultures, Korean culture is different enough that some adaptation will be necessary. If you are unwilling to adapt, you might want to rethink coming into an Asian culture for a year (or more).
The key thing to remember is you just have to roll with it. If they say or do something you feel is rude, don't get angry. They don't mean it that way. Half the time it's a language barrier, and it's harder for Koreans to find the right words, so bluntness prevails. The other half it's just how they're used to interacting with each other. It can sometimes be frustrating if you let it, but it's a different culture and you just have to go with it. The more you fight things, the more uncomfortable and unhappy you will be. If you let things go or laugh things off, any cultural differences you face will be much easier to handle.
4) Be Willing to Try New Things
You're coming to a different country with a different culture, different food, different beliefs, different everything. And they're pretty proud of most of it. If somebody offers you something, like food, unless you have an allergy--at least try it once. If you try it once and don't like it, then you can say that the next time it's offered. But if you're coming for an experience, make sure to... you know... experience things.
The food is different than you would expect. Most of it is actually pretty good, even if it doesn't look like it. I'm a pretty picky eater, but I at least tried most things (I did draw the line at octopus tentacles, though). I found the trick was to try it before asking what it was. I discovered I actually enjoyed some things I normally wouldn't have touched in a million years. But don't feel that all the food here is like bugs and tentacles, though. The majority of food is along the lines of fish, chicken, vegetables (including various types of kimchi), tofu, noodles, rice cake, and regular rice (until you're sick of it). There are two flavor types here, too: bland and spicy. I personally preferred spicy, especially the soups that have a red pepper paste base. The unseasoned, usually tofu-based foods were not exactly my favorites.
But food isn't the only thing to try out. There are also various cultural experiences (or otherwise, like adventuring, even if you're not an adventurer) you should be willing to go out and try. Though most of them are better with friends, which leads me to...
5) Make Friends (But Be Picky)
It's important to make friends here or else you'll be quite lonely. And you'll end up getting strange looks if you go places alone. Korea is made for friends and/or groups, and Koreans rarely do things alone. If you can make a Korean friend, even better.
But be careful. Not all foreigners are the best people. Most of them are, but you do have those who are here because they're running away from their problems back home. You have the Negative Nancy's who just want to complain the whole time. You have others who just want to be here because of Drinking Culture and want to do nothing but go out partying and drinking all the time. (A lot of people you'd meet on a night in Itaewon in Seoul are going to be like this.) And if you're female, be careful with Korean guys. I've heard more than one story of proclamations of love and attempted kisses after knowing them, like, a day. (Again, they're not all like that.)
6) Find a Hobby (or Two)
Find something here you like to do and do it. It'll help keep yourself busy and help you acclimate to living here better. Take a class. Join a gym. Try a martial art. Join a club. There are also a handful of websites out there for foreigners to do and see great things together, including but not limited to Adventure Korea and meetup.com.
I joined a gym and practiced Muay Thai for a couple months before I fell into a string of sicknesses and had to drop out. And I did other things, too, like river rafting and other site seeing. (Which ties back into the "Try New Things" suggestion.)
7) Learning Korean Isn't Necessary, But It's Helpful
Basically what the title says. You can survive living in Korea without really learning to understand or speak the language... but it helps. You will, whether you try or not, pick up a couple dozen words while you stay here. But I mean a little more advanced, like sentences, questions, responses. The language barrier is by far the most difficult thing living in Korea, so any way to lessen that gap can be helpful. Again, you can survive without it, but it's easier if you know some basics. Also... if you don't start learning before or soon after you get here, you never will. That's not just from personal experience, but basically from every person here I've ever talked to. If you don't bother learning anything by the time your first month comes to a close, you'll probably never try.
8) Don't Believe Everything on the Internet
I watched hours and hours of videos and read probably a hundred articles before coming to Korea. I'd say maybe half of what I learned was false. And a bunch of the stuff you read is from disgruntled former teachers who landed a painful job at a Hagwon, or people who refused to adapt to the culture, or people who just shouldn't have come to a foreign country to begin with.
Other things like "you can't find ____" isn't always true. If you know where to look, you can find almost anything. Of course there are exceptions, but most practical things you can find here in some form or fashion.
9) There are Four Very Distinctive Seasons
They are quite proud of this fact, too. But know that in summer, it's very hot and very humid (and coming from a southeast Texan, this says a lot). And their air conditioning is probably not what you're used to. The trains and supermarkets and whatnot are nice, but things like schools and your apartment or officetel will sometimes be out to kill you. And because you most likely will not have a dryer with your washing machine and instead have to hang-dry your clothes, expect it to take longer due to the high humidity. Also be ready for the rainy season, wherein it will rain about every other day for two weeks and then every single day for another two weeks.
On the flipside, in the winter, it gets very cold and sometimes a bit snowy and icy. Most inside places are well enough equipped to deal with the cold... again, the biggest difference being your school. It will most likely have to follow a city-wide ordinance to save energy and, therefore, not turn on the heat. Just like in the summer when you have to teach while dying from heat and lack of hydration due to it being sweated out of your body at profuse levels, you might have to teach in the winter with a bit of frostbite.
In the end, though, your body will eventually acclimate, but make sure you have appropriate clothes for both. For the winter, have your down coat and layered clothes. The cold lasts from about November to April. For the summer, have appropriate clothes to stay cool; ladies, you can show all the leg you want (seriously), but keep in mind cleavage is a bit of a no-no here unless you're trying to sell something (your body or otherwise).
10) North Korea?
Unless South Korea is worried, you shouldn't be. During the whole big thing last year where Americans were freaking out about North Korea? You barely heard a peep from South Koreans. Frankly, they weren't worried at all. During your stay, the only times you'll probably hear about North Korea is if the west is worried about North Korea and you're reading western news. Or if you bring it up first in conversation. It's not really a touchy subject. They just aren't afraid and know the North is nothing to worry about. And until they do worry, you shouldn't.
Well, I hope anyone reading these found this post useful, and I hope your Korean adventure is a fun experience for you!