Wat do you do wen you have a nervous habit?

i have this wierd habit of getting nervous like...really easily - and im not even talking about like just being normally being in front of a crowd, i literally CANT put myself in front of a crowd, and the sad thing is, im still in high school, which is literally going to FORCE me to BE in front of a crowd until i graduate.... i really dont know how to help it and its affecting my grades, my attitude, and even changing my personality.... i really need help, i dont kno wat im supposed to do about this....like in the middle of cl i try as hard as i can to be all calm and all until all of a sudden when it gets all quiet i just suddenly start shaking and jolting and i dont even kno wat to do anymore.... i also tend to sweat alot when it gets real quiet and im never sure if i breathe too hard.... cuz i get like so easily nervous and theres like NOONE around me that does that.....its so embarring ooooh please help me....this is really affecting my grades, and causing me to skip and arguing with my parents alot. i really am in a situation where im lost and going under depression. any advice? thanks for your help...