Avoid glare from windows or overhead lights. Close the window shades to stop light from hitting your computer screen, and position your desk away from direct, glaring lights. Make sure the room has proper air circulation.
Your monitor should be at arm's length from your body with the top of the monitor at eye level so you are looking slightly down when you are working. Many new flat-screen monitors are easily adjustable. When typing, your arms should be positioned so that your wrists are not bent when you type. When using a mouse, make sure it's not too high or it will strain your wrist. Use a mouse mat with a pad as support.
Your desk and chair
Adjust your chair so your back is well supported, from top to bottom. Use a lumbar support, if you need one. Sit upright and don't slouch. When you are typing, your elbows should be at 90 degrees or slightly less. Your feet should reach the floor (use a footrest, if needed) and your thighs should be parallel to the floor with your knees bent 90 degrees.
Look away from the screen from time to time to rest your eyes, and make sure the font size you use is big enough