Really scared of rollercoasters!!!?

My best friend is having me over to go down to St. Louis. I really like him and I want to go, but he said we were going to six flags where all the roller-coasters are. I am scared to death of them, I wouldn't even ride those dinosaur rides at disneyland. I really want to go, but I don't want to spoil it. And he's having otter friends I dont know over too. And if I chicken out, they won't like me. What should I do or say! I don't k ow what to do