If you see reflections or glare when the monitor is turned off, pull down window shades, move lights away from the reflection or tilt the monitor up or down slightly. Ideally, you should sit tilted forward slightly so your knees are lower than your hips. This is simple with most office chairs, but the chair you have at home is probably a different story. If you're using a hardback chair, at least get a thick seat cushion or firm back pillow to support the lumbar region of your back. Your feet should be flat on the floor or on a slightly elevated footrest.
Posture is important.
Your arms should hang straight down from your shoulders, with torso and wrists straight and the angle at your elbow 90 degrees or more. To avoid bad posture habits, take frequent stretch breaks. The keyboard tray should be titled away from the body, with the side closest to you slightly higher. If your keyboard can't be raised, try wrist pads. The mouse should be kept close to the keyboard and at the same level to decrease reach.