This study will examine the safety and effectiveness of the medicine metformin to help overweight children control their food intake, weight, insulin, cholesterol, and triglyceride (blood fat) levels. Obesity and high insulin levels can lead to high blood pressure, diabetes, high cholesterol and triglyceride levels and heart disease. Metformin-approved by the Food and Drug Administration to treat adults with type 2 diabetes mellitus-helps lower insulin levels and may control weight gain in adults.