The thing you sit on, chair, is the most important for your health. The chair must provide you opportunity to adjust yourself according to the position of computer. The angle between your legs and backbone must be 90 degrees which means that your back must be straight.
The distance of computer from your eyes should be at least 50 cm. The effect of rays emitting from the screen of computer can be minimized through proper distance.
Your arm must make an angle of 90 degrees which means that the computer should be in line with your elbow. This will minimize the fatigue you get after using computer.
This tip applies not only to desktop computer but also to laptops and MAC books. Furthermore, if you are using laptop then you can facilitate yourself by using external keyboard and mouse.