Why do i feel like im going to pass out when i get really nervous?

when i get really nervous about something i feel realy lightheaded and think im going to faint. i grab for something so i dont fall. it happens when im walking toward some1 that ik. at the hallways at school. or a classroom of ppl. its when im walking in and i twitch and think im going to faint. idk y i get nervous but i do and i have a fear that im going to have one of these attaks and its realy embaressing. please help!!!! why does this hapen to me and hwo can i make it go awway imedietely?????