Korean is very heavily based on seniority and politeness, as many of you probably already know. Therefore, there are many variances of speech depending on the person(s) you're addressing.

Typically, it's safe to say that adding 요 (yo) or 다 (da) to the end of a sentence/word makes it polite.

Almost always, Koreans will attach endings to the ends of names, which depends on level of seniority, gender, role, etc. These are called Honorifics.

Some common ones are:
씨 (ssi) ; a common formal ending
님 (nim) ; probably most formal, usually for a boss/teacher/etc
야, 아 (yah/ah) ; mostly for close people or kids. You should be given the OK before calling someone that
선배 (sunbae) ; someone older/more experienced than you in work or something. (if its obv you're talking to him, then u can just replace his name with that)
누나 (noona) ; what a guy calls an older woman he's close to
언니 (eoni) ; what a girl calls an older woman she's close to
오빠 (oppa) ; what a girl calls an older guy she's close to
형 (hyung) ; what a guy calls an older guy he's close to

Common Phrases:
Hello/Good morning (formal) = 안녕하세요! annyeong haseyo
Hello (casual) = 안녕 annyeong
Good Morning = 좋은 아침 joh-eun achim
Good Night = 좋은 밤 joh-eun bam
Good Night (my preferred method)
Formal = 잘자요 Jaljayo
Casual = 잘자 Jalja


What are you up to now? 지금 뭐해요?
Common Answers:
Not much - 그냥 있어
Studying - 공부하고있다
What about you - 너는 뭐해(요)?

Nice to Meet You
Formal, but for people closer to your age: 만나서 반가워요 Manaseo Bangawoyo
More formal version: 만나서 반갑습니다

Thank You
Formal: 감사합니다 Gamsahamnida
Semi-formal: 고마워요 Gomowoyo
Casual: 고마워
Very Casual: 고마