--Try some of the wilder but mostly-in-one-place rides, first. If you can handle rides that go high into the air, coasters should be the next logical step. The speed isn't nearly so off-putting as the height of the tracks can be, at times.
--If the ride has a seat/car model out in front of it that will let you sit down in it and test the restraints, do it. Have two friends stand on either side of the overhead collar and pull it against you, then try to squirm out of it. Then realize that the locking system in the cars is probably a hell of a lot stronger than they are. Part of overcoming your fear will lie in convincing yourself that you'll be safe during the ride. If it would help, do some searching around online and check out reports on accidents that have taken place at amusement parks. Nine times out of ten, you'll probably find that the accidents happened because the rider or riders injured tried to do something stupid (like stand up during the ride, undoing the safety belt, etc.).