Based on the level of respect or formality, you can say "I'm American" in Korean in 3 ways:

• Informal: na-neun migugin-iya (나는 미국인이야.)
• Formal: na-neun migugin iyeyo. (나는 미국인이에요.)
• Very formal: jeo-neun migugin-imnida. (저는 미국인입니다.)