The top of the screen should be no higher than eye level. Lower heights may be required if you wear bifocals. Use a document holder to avoid unnecessary neck and shoulder strain. Have a lamp nearby as well. This has nothing to do with the physical environment of your workstation, necessarily, but you should breathe deeply when working at the computer.
Shallow breathing creates tension and fatigue.
Breathe from the gut and fill your lungs. Take a 10-15 minute break from keyboarding every hour. Do what is most comfortable for you. You'll be able to tell something needs changing if you experience neck strain-or any other strain for that matter. Okay, so you're experiencing pain regardless of following all of my suggestions.