When you build muscle it doesn't replace the fat. If you want more muscle definition you need to eat right and burn the fat through a cardiovascular exercise (jogging ect.). Stick lo low gi carbs like oatmeal, sweet potato and brown rice and get your protein intake from egg whites, steak, chicken breast and fish. Do cardio straight after weights, or on an empty stomach in the morning before brekfast. Start with 20 mins 3x a week. If you are going to use supplements start with whey/casein protein, flaxseed oil, multi-v and some psyllium husks. Would't worry about spending money on supplements to help you burn fat yet, give that a go and if you want to get more serious later then try a fat burning pill as none of them come close to giving the results that they promise.