After reading some of the previous answers, they are very solid answers. However, there is one more thing I would like to add that really helps. I'm 18 and love roller coasters, Always have/always will. I have a younger friend who was absolutely terrified of roller coasters. So I've gone to a few amusement park websites, along with YouTube, and watch the POV (point of view) videos. It shows you the entire ride from the front of the ride, so if you put it in full screen and sit relatively close, it almost feels as if you are on the ride.
It is a great way to start to see if you'd be interested. From there, go to an amusement park with a wide variety of coasters, and start small. I started my friend off at Coney Island in Cincinnati, Ohio, where they have only one coaster; while it's a relatively small ride, it has the whiplash effect of most rides. He was thrilled by it and we've obtained season passes to Cedar Fair's amusement parks (Kings Island/Cedar Point, etc) Roller coasters are great fun and I hope you get to experience all of the enjoyment that we do!