How to keep calm during a convo ?

How to keep calm during a convo ? So I suffer from this stuttering problem. Thing is that I don't stutter around everyone.... I don't stutter when I am with people I am comfortable with, like my cousins, my best friend etc.. But, when I am talking to my classmates, or someone new, my mind always starts rushing ahead, thinking about how the other person will perceive me, and what will they say and so on.... Due to this imbalance between my brain and mouth, I find myself stuttering. How do I keep calm?.