What can I do to start regaining muscle right after my cast is removed?

Therapy. Ask your doctor how much you are allowed to do immediately after a cast is removed - bone may still be healing. Start with range of motion/stretching exercises. As you start regaining your normal motion, you then can start specific strengthening. As you use your limb more, the muscle strength/bulk will return over time.
Rehab. Ask the doctor who had the cast removed. Rehab is determined by the type of injury, the treatment you had received and by your remaining problem. It is possible that range of motion is more important than muscle bulk at this stage, and the treatments are different.