Scared of rollercoasters? HELP?!?

Okay my cousins are planning a trip to Thorpe Park, and I'll admit, I'm absolutely terrified. Enough to tremble just watching the videos on YouTube. I don't want to look like a wimp, and I don't want my entry fee to go to waste. I just want to have a good time, but I'm dreading every minute of it. I've only been once, and I only sat on Tidal Wave. It wasn't so bad, but the rest seem so... damn! Any tips and reassurances?