don't make your joints do the work. relax all your joints and through your body as much as possible and do the pulling and pushing motions and let your muscles do the work. it's tempting to just let your arms and joints do the work but you will feel it in the muscle groups you are working on when you're doing it right and they will get physically hot to touch as they fill with blood. don't rush the movements, if you're using momentum to get the weights moving you are using too much weight - focus on form (smooth, even, controlled movements) and the strength gains will follow. don't be too ambitious, start with low weights and work up slowly. keep a straight posture with your tailbone tucked in and don't fuck up your joints, you'll regret it!!
Actually, losing body fat leads to muscle definition, the extent of which, in the absence of significant fat, is dependent on genes, as is muscle size and strength. High weight and fewer reps yield mass, but, more importantly, strength.
Low weight, high reps engage the slower twitch, small muscle fibers, which are used more for endurance (i.e., aerobic) activities. High weight, fewer reps engage the larger, fast twitch (2X) fibers, which tend to be more anaerobic. These work well for strength related tasks but fatigue quickly.