===== Formal ======
Hello = 안녕하세요 [an-nyeong-ha-se-yo]
Good bye (when you're the one staying) = 안녕히가세요 [an-nyeong-hi-ga-se-yo]
Good bye (when you're the one leaving) = 안녕히계세요 [an-nyeong-hi-gye-se-yo]
===== Informal ======
Hi = 안녕 [an-nyeong]
Good bye (when you're the one staying) = 잘가(요)* [jal-ga-(yo)]
Good bye (when you're the one leaving) = 잘있어(요)* [jal-iss-eo-(yo)]
* You add the 요 to be polite, you can drop it when talking to friends or people younger than you.