Sitting at a desk can be bad for you but there are simple steps to help you survive.

Ensure the centre of the screen is at nose level, Reduce reflection on the screen by using blinds or additional lighting,  

Fore arms to be parallel to floor ,keep your elbows close to your body, mouse and keyboard  to be at the same height. Keep your wrists straight.

Regularly look away from the screen and focus on the distance. Have breaks to stretch arms out behind you by linking hands and raising and lowering arms.