Diet is what cuts you up, with a staple of heavy lifting and cardio.
thats what you need to concentrate on, but while doing weights, decide exactly what your goals are, because you will need to make a choice between size/strength, or losing weight.
and the way to train and diet will reflect this, so lifting heavy will burn calories, but while eating a calorie defecit diet to lose fat, you won't be making big results in the size or strength department, so just be prepared to not see huge results in that area, but from personal experience, after eating a well structured diet, + exercise, i have always preferred to lift heavy and i have seen results when i start cutting up.