Acanthosis nigricans looks like velvety dark skin. It occurs in the skin folds at the base of the neck, the armpits, and groin area. It might be a sign that your body is producing too much insulin as a result of being overweight or having a diet with too many "high-glycemic" foods, especially if you have a family history of Type 2 diabetes. Acanthosis nigricans is not dangerous in itself and doesn't necessarily need to be treated, but the underlying weight condition should be addressed by a nutritionist, who will teach you how to follow a low-glycemic diet.
One simple rule to following a low-glycemic diet is to avoid these "white foods" -- refined sugar, white flour, white potatoes, white rice, and regular (white) pasta. Instead, your diet should have plenty of fresh fruits (instead of sugary desserts), vegetables, and unrefined complex carbohydrates, for example, whole grain bread and pasta, brown rice, legumes (black & red beans, chick peas etc), and yams or sweet potatoes.
Anyone with acanthosis nigricans should see a doctor for advice. Start with your family doctor or pediatrician, and consider seeing a dermatologist for skin care advice, and a pediatric endocrinologist if your primary care doctor is concerned about pre-diabetes.