Based on the level of respect or formality, you can say "I need to see a doctor later" in Korean in 3 ways:

• Informal: na-neun euisa-reul bol pilyo-ga itseo (나는 의사를 볼 필요가 있어)
• Formal: jeo-neun euisa-seonsang-nim-eul bol pilyo-ga itseoyo (저는 의사선생님를 볼 필요가 있어요), jeo-neun euisa-seonsang-nim-eul bol pilyo-ga itseupnida (나는 의사선생님를 볼 필요가 있습니다)
• Very Formal: jeo-neun euisa-seonsang-nim-eul bol pilyo-ga itseupnida (나는 의사선생님를 볼 필요가 있습니다)

- I: na-neun (나는), jeo-neun (저는)
- to need: pilyo hada (필요하다)
- to see: boda (보다)
- to need to see: bol pilyo-ga itda (볼 필요가 있다)
- doctor: euisa (의사), euisa-seonsaeng (의사선생), euisa-seonsaeng-nim (의사선생님)

Please refer to How to pronounce Korean Alphabet for the English pronunciation in this answer.