Alton Towers is the best park to build up on your fears and face them, as there is a wide variety, and the coasters at Alton are something "less intense" than others of its kind.
Enter the park, start with Spinball Whizzer, try not to be intimidated by the drop and overbank as the track size tricks the brain into thinking the coaster is much larger, but it's hardly massive. If you're worried about the spinning, it is not disorientating at all so don't worry about that either. You should manage to ride it no problem, but if your nerves are pulling you away from riding, I'd recommend the Flume first, then return to Spinball. It's common among most people, your nerves are at it's worst if you haven't been on any ride on that day yet. Spinball should then get your blood flowing and adrenaline pumping.
Think then, you have just ridden one of the most intense coasters at the park.
Head to Forbidden Valley for Air. This is a very tame "thrill", so this is a good starting point for larger coasters. Don't be intimidated by the flying position, and transitions through the track, you are practically stapled into the train and you barely feel yourself moving. It's a very relaxing ride, and you'll soon be pulling the Superman manuevre as you coast round the track.
Next stop is Nemesis. This is made out to be a lot worse than it actually is, so you may as well ignore everything people say etc. It isn't actually THAT intense. The only thing you need to take into mind with Nemmie is that it's over-hyped in terms of intensity, and just think, you're only pulling a maximum of around 3.5G's, so it's not as forceful as people make it out to be. Make sure you do Nemmie as it will be essential to transform your fears into desires.
Take the SkyRide to Ug Land and get queuing for Rita! This is a step up from Nemesis and Air, due to it being a bit more "shocking", than "layout", very much like Oblivion too. The thing you need to think about here is 60MPH isn't actually that fast. It's only 10MPH faster than Nemesis, which isn't a lot at all, and the acceleration is paled in comparison to Stealth. You may have problems with Rita, the wait for the launch can send you a little panicky if it's a first time, but the actual launch is a breeze. If you are nervous, come back after you have been on a few level 3 coasters as I call them (such as: Nemesis, AIR, Nemesis Inferno, Shockwave, G Force etc).
After Rita you shouldn't have much problem with the others at the park, Oblivion and Ripsaw can be a bit tricky at first, but you will gradually overcome that fear.
Another tip is to get a brave girlfriend/boyfriend, and take her/him. You wouldn't want to look an idiot in front of that person would you