첫 means 'First' 멘션 means 'mention' We say it as it is pronounced in English ( If you can read Korean, I belive you know what i mean.) 주고 받다 means that '주다' is to give, and '받다' is to recieve or to get or to take. So, It can be 'give and take'. Therefore, It can be translated " We are giving and taking the first mention" as it is. BUT, I would say that 'We are talking each other for the first time' is more natural and appropriate. I don't know if you got it well. I'm worrying that if it makes you more confusing. Also, If there is something wrong with my English here like grammar or unnatural sentence, Plz let me know :) Thank you, I hope it will be helpful for you :)