When learning Korean, after learning Hangeul, the first thing you should learn is how to greet someone!
In Korean, 안녕하세요 means a number of things; "Hello", "Hi", "How are you"...
If you are new to Korean, it may take a few tries to say it correctly: "an-nyeong ha-se-yo".
It can be broken down into two parts;
안녕 [an-nyeong] means 'peace, health'
하세요 [ha-se-yo] is the polite form of the verb '하다' which means to do.
It is the generally accepted, polite way of greeting someone. The correct response to 안녕하세요 is 안녕하세요.
Well Done! You've just said your first phrase in Korean!