First few weeks, I was using the racket that the instructor gave me. I had a serious elbow pain for several days. My hand was shaking, I could not hold a pencil in my hand. Then I learned, it is because of several things:
-The racket I was using was too heavy
-The grip was too big for me
-I was holding the racket too tight
-I was only using my arm, rather than whole body
After about 4 weeks, my pain stopped. During this time, I bought a very light racket, lightest on the market I think. Prince -Air Light. 8.5 oz, Oversize, Length 27.5 inch. Great for beginners like me. I found the right grip size. Here is one of the web sites that show how to find out the right grip size. Second, I guess I've improved in holding and swinging right, and the problem is solved by itself.

During this first few weeks, I used Sportopedics Elbow Strap, it really helped me, especially because I needed to hold a pen.
Playing doubles is really fun. The problem is, how can I find 3 other people while I can't even find one person to play singles :)