Start by checking that your chair and desk are well adjusted:

Chair
	Adjust the seat height so that your feet rest flat on the floor.
	Make sure that there is no pressure on the back of the knees.
Desk
	Adjust the desk height so that your wrists can rest comfortably on it.
	Your forearms should be level with the desk top and your upper and forearms should form a 90 degree angle or slightly more.