On average a pound burns 12 a day. So simply pick the weight you want to be, multiply that by 12, and try to take in no more than that many calories per day. You'll eventually reach that weight. That number of calories is what it takes your body to keep running. So if you exercise too you'll burn more and lose weight faster. And don't take in any calories from liquids. Oh, and green tea is magical.

I accidently lost 10lbs one week when I started drinking it everyday. Besides helping in weight loss I noticed it cut my appetite.
This should help
a lot:
http://home.howstuffworks.com/diet.htm