What specific type of injury? Sometimes, people often misdiagnose an injury just because of what it looks on the outside. There are types of injuries that still needs further examination such as x-ray and even CT scan (especially for head injury). So it is always best to seek advise from the experts. I suggest that you go to your family doctor to have it checked.
No. it will just make it worse since they did not undergo any professional training. They also do not know the depth of the injury as compared to a doctor/physical therapist. I suggest that you go straight to a doctor depending on the severity of your injury. You may opt to go to a rehab doctor, a physical therapist, or a surgeon.