How many calories to lose weight?

I would like to know about the calorie intake to lose weight. Here is my story. I am 35 years old. Four years ago I weighed 125 lbs, I am 5'6". I quit smoking and had a child and now I am 190 lbs. I have tryed every diet out there even Phentermine pills which made me sleepy. I have had my tyroid checked and it is normal. I am also did 1200 calories and 20 fat grams a day (very strict) and it did not work....HELP...I need to get this weight off. I have tryed ATkins and it does not even work. I dont know what else to do...Please Help...