How to stop elbow pain?

Stop playing tennis. Elbow pain would usually be caused by repetitive movements of the joint, causing tendonitis (inflammation of the tendon), hence the name, "tennis elbow." so reducing activities that exacerbate or cause pain should eliminate the problem. There are other causes of elbow pain that should be ruled out, if you cannot localize or discover a cause on examination. See your doctor to make sure!
Ice. Apply ice 3-4 times per day, for 20 minutes at a time. Take an over the counter anti-inflammatory medication if you can.