Weight loss issue

I really really wanna know how all of you girls lose your weight. Me and my sister have been on a diet for a while now and I have gotten no progress. I walk more and I lift weights. I drink nothing but water and occasionally milk. I cut all the sweets and pop (I haven't had pop in over a year) and nothing works. I don't understand why. I don't know if it has to do with it, but I get headaches like you won't believe... help? I currently weigh like 265 and I really wanna be down in the 100 area. Maybe 160-180. I don't know. I just need help and advice/tips.