You better start up with the positioning of your desk chair. Make sure you adjust your chair in a manner that your feet are flat and your thighs are parallel to the floor. You can always adjust the height of your chair according to the desk you use and your own comfort.
It is important that you always your feet flat on the floor. If you use a higher desk chair, you can place a step-stool or a box to rest your feet on.
Your desk has to be adjusted according to the comfort of your arms. Ideally, your elbows should be very close to 90 degrees angle when you place them on the desk and your shoulder should not raised from their mean position at all. If you feel you have to raise your shoulders while working on your desk, you can reduce the height of the desk or increase the height of your chair to tackle the problem.
The computer monitor should be at the level of your eyes. You must not have to bend your neck downwards for looking at your computer screen. You can place a box under the monitor to bring it to your eye level.
Modern optical computer mouse does not need a mouse-pad and you better throw it away from your desk. You should also practice using the computer mouse with both of your hands alternatively.
Ensure that your wrists are absolutely straight, not curled or bent, while typing. You can use a gel wrist pad with your computer’s keyboard. If you feel the keyboard is a bit away from you, you can place the gel wrist pad beneath your forearms as well.
You can also use gel pads while using the computer mouse, as it will help you to keep your wrist straight.