If you're already overweight, would it be okay to diet and start body building?

If approved by a Dr. Weight-lifting burns stored fat only if the activity is using lots of calories. If one lifts a heavy weight one repetition, and then walks around chatting with friends, then not much fat will be lost. In general, if a person exercises more than he did before, but he does not start eating more calories, then his body will use stored fat to provide the energy the exercises require.
Overweight. Get a diet plan from a dietitian and an exercise plan from a trainer. Avoid androgenic steroids.