To be seated properly in your chair your feet must rest flat on the floor and your thighs parallel to the floor You should use a footrest if your chair does not adjust low enough or if your work surface is too high.
Chair
The lumbar support on the back of your chair should fit into the small of your back and should match the natural curve of your spine. If the chair back is adjustable forward and backward, adjust the angle to what is comfortable for you.
Once you have your chair and work surface height adjusted, adjust your computer monitor so the top of the screen is at or just below eye level.
Try to keep the mouse as close to the centerline of your body as possible to minimise the amount of reaching you have to do.
If you type and reference material from paper you should consider using a document holder. Place the document holder at the same distance and height as your computer monitor.
It's very important to take a break from working at your computer every hour. Micro breaks can be taken every 30 mins.