the best way of building muscle and losing fat at the same time is by doing anaerobic exercises , its a very good way of converting the extra fat in muscle. it involves activity like hill climbing , weight lifting , strength training, skipping. besides that doing any other physical activity would have a similar effect bt the rate would vary. also you would need to get rid of all the unwanted carb from ur diet , avoid potatoes , banana , sweets,, fried foods . and eat healthy protein foods like egg white, milk , panner , lean meats .