How much weight would I lose?

I'm 17 years old, 5'7'', and 130 lbs. I want to lose at least 20 lbs by September (when school starts). If I walk briskly in the morning (before eating breakfast) for an hour everyday and eat less, how much lbs do you think I would lose? Please suggest an alternative plan that would help me lose 20 lbs if you think my plan is ineffective. I appreciate all helpful answers. Thanks! :)