1) sit on your chair so that so that your feet touch the ground comfortably. For the vertically challenged, if your office chair doesn't go low enough for you feet to comfortable contact the ground without weird angles, get an adjustable angle foot stand.

2) make sure your keyboard and mouse are close enough to you, and that your chair is close enough to your desk, that you aren't forcing yourself to hunch over to type or mouse. bend your elbows at 90 degrees beside you, with your arms forward as if you would use it. This is the most natural and least damaging long term arm position. Set your adjustable arm rests to this height.

3) This or slightly lower is an ergonomic height to minimize weird bending in shoulder, elbows, and wrists. This is roughly your optimum desk height. For most people, non adjustable desks are way way too high to be ergonomic.

4) you can consider dropping the height another 1/2 inch ("slightly lower") and get a wrist rest for your keyboard and an arm wrest for your mouse, which minimize wrist bending if set up right and support your arm intermediataly, but these are not for everyone.

5) If you're really into preserving your long term health, purchase a vertical mouse or scroll mouse to use during non gaming times. They take some getting used to but then you aren't always using the same muscles in the same position. The goal here is to vary the use.

7) the top of your monitor(s) should be roughly around your comfortable eye level so you aren't looking up.

8) Stretch your wrists and forearms every hour or two. One stretch I like to do that has helped me with my tennis elbow is veru simple. Stand up with room in front of you to swing your arms. Hold your arms all the way down with your elbows straight or near straight and your fingers facing forward, palm down.

Slowly swing your arms forward and up, keeping your fingers facing forward.

As you get to the top of the arc, tilt your wrist forward and curl your fingers. No need to make a fist.

Hold for 10-30 seconds where it feels like its stretching in your inner and outer elbow, and in the tendons around your hand and wrist. Repeat a handfull of time a day while working on a computer.

9) If something is hurting, this is an injury. Figure out what's causing it and treat the cause, not just the injury, or it won't get better. Consider reducing computer usage or activities with repetitive motions to let your arm recover.

10) If the injury is accute (preferably before then, but definitely if it is intefering with your day to day life) consider seeing a doctor. if you have health benefits massage and physiotherapy are both very helpful along with activity reduction. A doctor may recommend temporary use of NSAIDs topically or as pills, or in more serious cases direct injection of anti inflammatory steroids like cortisone. In cases with severe existing damage, they may recommend surgery.

Look up some guides and get a desk you can adjust - even if with difficulty so you only set it once - or something fancy with motors and control panel. Spend money and time on the ergonomics of the things you use all the time - chair, desk, monitor, keyboard, mouse. Your body will thank you for it.