When they are sitting next to you, smile and say hi, tell them your name. "Hi, I’m (insert name), how are you doin today?" If a girl, say hey I really like your earrings, shirt, were did ya get it? If your in class make small talk "man this class sucks or this is so boring". When you talk look them in the eyes (even if you blush) just have a conversation. If you have to pretend they are a sibling or someone you feel comfortable with and just talk.

It will take practice but eventually you will overcome most of it.
References :

Try meditation?

Try sports?

Try martial arts?

Try yoga?
References :