Muscle Building and Fat
Fat does not make you fat per se. It contains more calories per gram than carbohydrate or protein but, without being consumed in excess, is unlikely to result in fat gain.
Essential fatty acids, such as omega 3's and 6's, are required for many biological purposes, such as hormone function. Do not exclude fats from the diet, as indicated above. However, focus on the polyunsaturated and monounsaturated fats over saturated and trans fats.