Based on the level of respect or formality, you can say "Guess what!" in Korean in 3 ways:

• Informal: mueot-inji chucheuk haebwa! (무엇인지 추측 해봐!)
• Formal: mueot-inji chucheuk haebwayo! (무엇인지 추측 해봐요!), mueot-inji chucheuk haeboseyo! (무엇인지 추측 해보세요!)
• Very Formal: mueot-inji chucheuk haebosipsiyo! (무엇인지 추측 해보십시요!)

- guess: chucheuk (추측)
- to guess: chucheukhada (추측하다)
- what (subjective): mueot (무엇), mweo (뭐)
- what (objective): mueot-inji (무엇인지), mueot-inji-reul (무엇인지를), mweonji (뭔지), mweonji-reul (뭔지를) - reul can be omitted.

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