The textbook response to 감사합니다 would be 천만에요, but it's not used a lot in every speech anymore. Many Koreans would just say "네" "아니에요" 아닙니다"

Formal: "천만해요" pronounced "cheon-mahn-haeyo" Informal: "천만해" pronounced "cheon-mahn-hae" However, this isn't what is always used. Not many people use this phrase. If you want to say it more casually, you can say... Formal: "아니에요" pronounced "ah-ni-eh-yo" Which directly translate, "It's not it" but it can mean "That's okay" "Don't mention it." So people use this to say "You're welcome." Informal: "아니야" pronounced "ah-ni-ya" Same as above.