First check that your desk, chair, and computer or other equipment setup is at a good level for your height and posture.
You ideally want to be looking at your screens straight on with a slight downward angle so you are not craning your neck to view any computers or monitors. Your arms should have a relaxed bend at the elbow that allow you to lay your forearms and palms at level height with your tabletop or a keyboard tray. You can typically adjust your chair up or down to achieve this, though you may want to try a cushion, laptop stand, or monitor riser. If you have the option, you may want an adjustable height desk or looking for a desk that is a particular height. An easy way to test this out is to use books, boxes, or reams of paper you already have at home to rearrange your desk.
Your feet should comfortably rest flat on the floor from this position. I am short so mine never do, and you can use a box or a foot rest for this. In full transparency, I used this the least in the office and no longer use one working from home (my feet often rest on the spokes of my chair).
You want your chair to be comfortable if you’re spending hours and hours in it! Office outlets or FB marketplace are good places to find discounted professional chairs. A less expensive alternative to a new chair is a good seat cushion. I use a seat cushion to make myself a little taller for my desk and to keep my back from hurting after sitting all day. I have used this cushion at work for 5 years and more recently bought another one (different brand) for home. These are not the cheapest options available (just under $40 at time of posting) but are highly rated memory foam options, cheaper than most new chairs, and long lasting. The two I have are still in great shape at 5+ and 1+ years respectively.
Standing desks are also very popular to promote health and reduced back pain associated with sitting. There are standing desk add-ons to move up and down, and entire desks that are adjustable. In an unpopular opinion, I personally dislike standing desks since I tend to get knee and foot pain from prolonged standing.
Other accessories I like for workspace comfort are the gel pad wrist rests for your keyboard and mouse. If you’ve spread out your workspace a good external keyboard will provide you a better range of motion. I also highly recommend blue light glasses if you do a lot of computer work. I don’t notice them proactively during the day, but do notice my head and eyes are more fatigued on days or afternoons I forget to wear them.
My most used, favorite items are the blue light glasses, gel pad wrist rests, and seat cushion. These are fairly inexpensive and I feel such an impact from them that I actually purchased an extra of each to have at home versus in the office. The most important thing is to find what works for you!
Note: This is my personal opinion and recommendations. You should consult a medical professional to address serious pain, injury, or fatigue.