The commonly held belief that to get lean you should do lots of aerobic exercise and lift lighter weights for more reps is, for the most part, a myth.
I'll put forward a couple of suggestions, depending on your goals, and then explain more about them:
To get lean - lift heavy weights.
To get big - lift heavy weights.
Think about this for a second - when (if) you go to the gym, you tend to see the same overweight people slogging away on the treadmills and elliptical machines and the same buff guys in the free weights area. I'll explain why:
1 pound of fat contains approximately 3500 calories. Therefore, to lose 1 lb per week, you need to have a daily deficit of 500 calories. Typically, a person of average fitness will burn about 5 calories per minute during exercise, say running. Therefore, assuming their diet stays the same, they need to be running for almost two hours every single day of the week to lose that 1 lb of fat. Ouch. Take your typical gym-goer. They do, say 45 minutes on the treadmill three times a week. They will burn about 700 calories a week from this, leading to a fat loss of 1/5 lb. In all likelihood, they'll be exhausted from that 45 minutes and perhaps reward themselves with a slightly larger dinner. It could quite easily be an extra 250 calories, immediately undoing that 45 minutes of work.
My point is two-fold:
1) Exercise burns very few calories. Our bodies are just too damned efficient.
2) It is much easier to achieve a 500 calorie daily deficit by eating less than it is from doing more exercise. Therefore, the diet (see previous entries on calories and protein) is the key.
I'm not saying that (aerobic) exercise is bad. It isn't - it's great for cardiovascular health and just for feeling good. But it's not necessary for fat loss. That comes from the calorie deficit from eating less.
Lifting weights on the other hand is extremely beneficial for changing the body. Most people want to be slimmer, some want to be bigger and more muscular. In both cases, muscle definition is important. What determines whether you get slimmer / leaner or bigger / more muscular is not the weights you lift, but the food you eat. Adding a little more detail to the previous suggestion then gives:
To get lean - lift heavy weights, eat plenty of protein, eat less calories.
To get big - lift heavy weights, eat plenty of protein, eat more calories.
If the calories are not there, your body simply won't build muscle. It will burn fat. By all means, do some running, rowing, elliptical, whatever - it's good for you - but don't expect it to do much for fat loss.
I'll go into more detail into the sort of exercises to do in a future post, but for the moment, I strongly suggest switching to weights in the gym, even if they are machines. Lift as heavy as you can for no more than 10 reps and 3 - 5 sets. Do this 3 - 4 times per week for no more than an hour each session.