As others have said, first make sure your posture is correct, keyboard height is correct, you are looking straight out or just slightly down at your monitor, etc. What I did is put my laptop on the desk, blanked the laptop screen, and put an external monitor on a tall clear box that was designed for displaying dolls (I had to order it). Some monitors now have height adjustments.
Next, make sure you are using good, relaxed keyboard technique. I play the piano and the same concepts apply to that. The first rule is, do not stretch your fingers to reach keys. For example, many people hit Backspace with their right pinky and leave their other fingers on JKL. This will cause tension in your fingers, hand, wrist, and forearm. Instead, rotate your wrist to the right and use your ring finger for Backspace. When you do this, your index finger should land lightly on the 0 key. Why not use pinky for Backspace? Because for me at least, that would require moving my entire arm, even involving my shoulder, whereas I can easily reach it with my ring finger just by moving my wrist.
Same principle applies for typing numbers: do not stretch individual fingers up to the number keys and try to keep the rest on the home keys. Instead, move the correct finger to the key you want to hit and let the other fingers follow along in a relaxed way. For example, when I type 8 with my right "tall" finger, my index finger lands between Y and U. If you try doing the same thing while holding fingers on JL; you will feel a lot of tension.
As an easy demonstration, hold your hands flat in front of you, parallel to the ground and relaxed. Wiggle your fingers fast and see how comfortable and easy that is. Now stretch your fingers out as forcefully as you can and try to wiggle them. Big difference!
Second rule is, make sure you wrists are not bent. Bent wrists make the carpel tunnel, the thing tendons and nerves run through in your wrist, smaller. This makes it more likely to cause friction and inflammation in your wrist as you move fingers.
Lastly, if you start to feel tightness, tension, fatigue, tingling, etc, STOP AND REST. Maybe run some cool water over your hands and wrists. Don't keep working or you will only hurt yourself.