Ah, it's a very loaded question because Korean grammar has many rules. However, the basics are...
verb politeness suffix & name/addressing someone suffix
Verbs:
ㅂ니다 / 습니다 - very formal (there is an honorific above this using "시" but ignore for now)
어요/아요 - polite
다 - plain style
어/아 - informal
Example of this, using 재미있다 - to be fun/interesting: 재미있습니다, 재미있어요, 재미있다, 재미있어
Name suffixes:
-씨 Polite, acts as Mr/Miss
-아/야 Used when calling someone's name rather than talking about them
-님 Honorific used for someone higher than you (teacher = 선생님, CEO = 화장님)