I've ridden roller coasters since I was 14. Ok, I'm 14 and 11 and a half months now and I'm gonna be 15 in 2 weeks. Since my first roller coaster back in the beginning of 2011, I got addicted to them and have ridden over 20 coasters and 36 fair ground rides so far. Like you, I was terrified of them, but afterwards I got used to it and it was more fun than fear.
Tips:
1- Don't close your eyes. The reason for this is that your brain cannot detect what is happening if you close your eyes and therefore does not prepare your body for the drops etc... on a roller coaster. This is at it's worst when a roller coaster does an inversion or corkscrew because you end up feeling like your going to throw up.
2- Scream and Laugh... or do a mixture of both! They both relieve stress and if you can actually focus on screaming and laughing then time will go by faster and the roller coaster will be over before you know it.
3- Feel the adrenaline (your heart racing). If you feel your heart beating fast due to anticipation and if you find that you like that feeling or danger then you will love the roller coaster!
4- Don't go on it if it doesn't feel right. If you see a gigantic roller coaster and think you might throw up on it then don't go on it! The chances are your caution is probably right and you will probably throw up.
5- Don't be over cautious! If a roller coaster is slightly tame and doesn't have major loops, inversions, drops and height then don't convince yourself that it isn't right when hearing people scream. People screaming can be intimidating but don't convince yourself you can't handle it when you actually can.
6- Hold on tight! Holding on as tight as you can and if you brace yourself they both help to make you feel more safe.
7- Go on with people at your level. If you can, but if you can't then that's alright to.
8- Try to avoid eating at least 2 hours before going on a major roller coaster... You have a 75% chance of throwing up if you don't.
Hope these tips help! And Don't forget the most important tip: HAVE FUN! That's the whole point of roller coasters, is to make you have fun... and being scared along the way is a part of it...
Enjoy your thrill seeking :D