How can someone who's been fat since birth lose weight? Aren't there too many fat cells already?

Lower caloric intake. Losing weight is conceptually easy but very difficult in the real world. If you can calorie count, the goal is to ingest fewer calories than you burn up. All diets do this but the hard thing is to stick to it. You will need nutritional advice, a doctor's help, exercise program and diligent removal of extra calorie food from your house. Sometimes behavioral therapy is warranted. Good luck.