Just keep your posture right. Get an ergonomically designed keyboard and mouse, or a wrist brace if those aren't preventing pain. You want to have a neutral position for your wrist, not tilting or twisting dramatically in any direction. Also, be sure to make sure you're relaxing your muscles from time to time (since you're new to PC gaming, you'll have to consciously watch this at first to make sure you're doing it. After a while it should become a habit.). There are also little exercises you can look up to do after a long session, if you feel it's necessary.