overweight

So I recently found out I am over weight for my height, I am 5'3 and 140 pounds. Of course when people look at me they don't see it. I hide it very well. My doctor said I am at risk for diabetes and heart disease and a lot of other things. I am only 24 years old I feel like I am too young to have all of these issues. In order to be healthy I would have to lose about 20 lbs. can anyone give me ideas on how to lose weight and become healthy?