Hi Laura,
welcome and good luck! That's probably do-able, sorry I'm not so good in imperial weights but I lost about 2 stone last summer in 3 months which is approc the time scale you're looking at. Remeber you also have to complete the (usually longer) Conso Phase afterwards.
Have you been to the official web site to calculate your true weight? That will also include a rough guide on time scales and in my case, it was fairly spot on. Don't worry, no need to pay or sign up, you just need to give an e-mail address.
Anyway - if you think you're ready to start, plan this with care and make sure you go shopping beforehand to have a fridge full of protein, the proper oatbran (not porridge oats, not oatmeal and not wheat bran!) and no temptations lurking in the cupboards (if possible)
Think about the attack days - I made sure I started AFTER a couple of social engagements that involved food and a stressful weekend when I would have had no time to think about organising my food! Sometime it's a good idea to wait a few days and then start when you have a few days where you can focus on the diet without having to juggle other factors like eating out etc.
Good luck and let us know how you get on!