I would say there are two ways of doing it. Either just brave it and hope that it doesn't scar you for life, or start with the weaker coasters first and then each visit build up slowly. If you are trying to conquer your fear at Alton Towers than I would suggest the following:
Start with Octonauts. As silly as that sounds, it's the perfect starter coaster for anyone big or small. If you are confident with that, then do Runaway Mine Train. Its a bit more forceful than Octonaughts but its still a very easy coaster to enjoy. Start at the front and then work your way back for an idea of intensity building. After you have become confident with that, then you have two options. Spinball Whizzer and Thirteen. They are about the same level of intensity and fun so are easily the next step up after Runaway Mine Train. If you can brave both of those, including Thirteens Crypt element, both towards front and back on Thirteen, than I suggest Wicker Man. It's definitely more fun and it is just an enjoyable ride. The front is not as intense as the back. Then I'd give Rita a go.. It's a lovely introduction to the more exhilarating coasters the park has to offer. With this, sit in the middle if you are nervous over the launch, You won't get the full force of it being in the middle. Than work to either the back or the front for the more forceful rides. After this, Galactica is your best shot. Its a smooth ride with an adding thrill to it. Oblivion is probably best next as it doesn't contain any inversions, but its scary the first time you ride it. On Oblivion there is no where to sit that is really any different. Its just as intense on the front row as it is on the back. After, its up to you. Nemesis is quite a forceful and intense inverted coaster, but it is smoother to ride, whereas Smiler is intense and forceful, but many find it easier to ride than Nemesis. Both are the most intense the park gets. Both are less intense at the front than the back, but it doesn't really make that much of a difference to me. To others it does though so it purely is different personal preferences.
Hope this helps