Best Diet to follow? 3 Stone to lose?

Hi all, I have approximately 3 stone to lose in 12 months ( for my wedding!) Firstly, is this achievable? Secondly, who has the best diet out there? There are so many to choose from, weight watchers, rosemary connelly, atkins, hollywood diet, etc etc! It's so confusing! My partner and I are joining the gym next month ( I would join this month - but i've just had an operation) Any tips or ideas ppl have would be greatly appreciated. I have hypothyroidism ( v severe - 300 mcg thyroxine a day) So losing weight is difficult, but it will NOT get the better of me!