At a desk and allowing your gaze to start at the top of the screen. Adjust your monitor height to meet this if you need to
The screen should be at least arm's length from your face and ideally have a filter to combat eye dryness and fatigue. If the text is too small to focus on comfortably, use the settings on the computer to enlarge it.
When seated, your elbows should clear the height of the keyboard. For desktop users, a separate slide out bench is ideal for the keyboard. For laptop users you'll need to adjust either desk or chair height.
Your wrists should be able to reach the mouse (if handheld) and keyboard without reaching your shoulders forward and you should be able to hover your wrists in a neutral or slightly cocked back position over the keyboard. If your hands drop into a flexed position, lift the props at the rear of the keyboard.
Consider gel pads for your wrists, swap the external mouse between each hand regularly and investigate different ergonomic mouse options if wrist and hand pain continue to be an issue for you.
The seat pan (the part you sit on) should be deep enough to reach at least two thirds of your thigh length toward the knee. Any less than this puts pressure on the blood vessels in your legs.
The seat back should support the natural curve of the lumbar spine. If you can't customise this, look for a chair with a height adjustable back, or find a contoured back rest you can attach to the chair back. A physiotherapist can help you select one for your size and shape.
Adjustable seat height is a must. Your feet should be flat on the floor and hips and knees should be at right angles when seated. Use a foot rest if your feet don't touch the floor when the chair is raised at the correct desk height.
A gas lift chair with 5 feet is more stable than a chair with 4 feet or less. If you're on a bare floor consider using a mat to reduce the rolling of the castors. You can get chairs with castors that brake when you stand up.
If you're using a sit-stand desk, make sure all parts can be adjusted to keep your neck and lower back in a comfortable, neutral posture.