Light weight

Low weight with lots of reps will help you trim down and stay lean. Low reps with high weight will build muscle and help you bulk up. Nutrition is really important too; a crappy diet will cut down on your results either way you to.

 

Lifting, with weights that you max out around 8-12 reps is good for building muscle, with a good diet of course. If your looking to cut down and look more lean, idk what to tell you. Even lighter weights with even more reps. But at that point you have to run cardio mainly.

Regardless, you have to do more than 20lbs if you packing muscle

 

If you are brand new at lifting, 20lb dumb bells is a fine place to start. You may find that you have to do more reps if 10-12 becomes too easy for you - and that's perfectly fine - you will still build the same muscle you would build with heavier weights, it will just take longer. Muscle is muscle - there is no such thing is lean muscle or otherwise, all muscle is lean. The amount of muscle definition you see depends on the amount of body fat you have, and body fat is mainly controlled by your diet.