You will gain weight as you exercise. Muscle weighs more than fat. If you want to do this the proper way, you must realize that you'll have to put away your scale and buy a tailors tape measure. You will lose inches...not pounds if you diet properly and exercise. However, for the first 6-10 weeks you will gain weight as you exercise, because you are converting fat to muscle. However, you will lose lot's of inches from under your arms, belly, and waist. You will need to exercise carefully, because if you lose too much weight from dieting your skin will become flabby and droop. That's why you must work on your "core" in order to strengthen key muscle groups (such as your abs, gluts, etc.). That way you will "tone" your body as you lose weight.
Take care :)