Ergonomics is "The applied science of equipment design, as for the workplace, intended to maximize productivity by reducing operator fatigue and discomfort. Also called biotechnology, human engineering, human factors engineering", as per www.dictionary.com.
For our purposes, we will not get into equipment materials or design, rather we will discuss setting up your equipment to minimize stress and fatigue and to maximize productivity and well being.
We assume that you read the section on Posture, and that your job involves being seated at a desk.
The ultimate goal is to have your body lined up in a neutral position, and to have your equipment adapt to you - not to have you adapt to your equipment.
Your feet should be on the ground; your seat height should be such that your hips are higher than your knees, as much as comfort allows; your arms should hang freely at your sides and your elbows/forearms should rest lightly on an armrest (remember, we are assuming that you are in a 'pelvic neutral' as described in the 'posture' section - do so before adjusting your armrests and seat height); your wrists should rest comfortably on a pad, but your wrists should not be flexed - they should be extended slightly, approximately twenty degrees; your eyes should face straight ahead or perhaps down a bit, and your line of sight should be at the center of your computer monitor or slightly above center.
Most people can achieve all of this with a standard desk and an adjustable chair, but if you cannot, you may need to purchase a foot rest, a stand for your monitor, and/or an adjustable desk.