The first thing you learn in any new language is the basic 'hello' greeting.
To say 'hello' in Korean you say 'ahn young ha se yo'
Written in hangul it looks like this: 안녕하세요
Here's a video to show you proper mouth movement and pronunciation:
Here's an awesome infographic from domandhyo
Hello in other Situations in Korea
Recently I've been learning about several other methods for greeting in Korea. Often times people will say "did you eat" after a meal but it isn't actually a question, similar to "how's it going". They really just meet to say hi. But usually the response is that you did eat.
To say this in Korean it's pronouced "Bab mugeo seo" or "bap meuku seo" its a bit difficult to write but the korean spelling is 밥먹었어.
This actually means did you eat rice?