I know you could type in "useful Korean phrases" into any search engine, but here are some really useful phrases that have helped me through the last few months:
*hello annyong haseyo
(general polite greeting said to anyone and everyone)
(very informal freeting used amoungst friends only~if ur not sure always use polite form)
*goodbye annyonghi kyeseyo
(when you are leaving)
*goodbye annyonghi kaseyo
(when other people are leaving and you are staying)
*thank you gamsam-nida
(you will hear this all the time! It is more of a "yes i aggree with what ur saying" kind of answer)
(again, it is more of a "no not really" kind of response)
*a little/small bit chogum
*how much is it? olma eyo?
(can be used to ask the price of anything~just point and ask)
*can I have?/please give me ........jus eyo?
(in Korean/Hangul is the oppsite to English so:
"Can I have some water" = "water can I have?" = mul jus eyo)
*sorry choesong hamnida
*I don't understand mot aladu lossoyo
*Could you take a photo of us? uri sajin chomchigo jushi gessoyo?
(but to be honest you can just use lots of gestures and the word "foto")
If your interested in learning Korean/Hangul, the Daejeon International Community Centre (situated Downtown, opposite the Daejeon Station) offers really great classes from beginner to high intermediate, as well as hosting lost of cool get togethers and events!
If your thinking of buying a phrasebook, I would highly recommend "Loney Planets Korean Phrasebook", which you can order off either Amazon, or Gmarket. I bought it off Gmarket for W10 700, and it arrived in 3 days, no hassle, no fuss! So easy and so incredibly useful!