Korean chooses a politeness level for every sentence. 존댓말 with the -요 ending is the safe everyday register for anyone you do not know well. 반말 drops the -요 and is for close friends and people younger than you - using it too early is genuinely rude.
Examples
- 어디에 가요?
- Where are you going?
- 어디에 가?
- Where are you going? (casual)
- 감사합니다.
- Thank you. (formal)
The mistake to avoid
Default to -요 with everyone. You wait to be invited into 반말; you do not choose it.
Where this fits in
인사와 자기소개, Korean A1: Greet someone, give your name, ask theirs, and say where you're from in Korean.