I have shoulder pain after playing tennis. i have been told this is a neck sprain. what should i do?

Stretch / PT / Yoga: Pain in a shoulder or arm can be the result of muscles or nerves being "pinched" higher up. We all tend to spend too much time on the computer (typing this answer!) - repeated use and underlying tension (not to mention disc disease) minimize the space through which nerves and muscles travel. Good "soft tissue work" either by stretching, getting physical therapy can help a lot. Yoga is terrific.

Shoulder vs Neck: You can either have one or the other or both. Tennis can put strains on your shoulder as well as your neck especially serving when you toss the ball up. The trapezius is a large muscle that travels from the base of your skull in the back of the neck to your shoulder area. Try doing some shoulder shrug exercises with weights on each arm. Also try and do some shoulder specific exercises.