Since a lot of folks are spending more time working from home these days and suffering from poor ergonomics, I wanted to share some tips on what your set up should be like:

1.  There is no one "right" posture.  Not just because everyone has a different body, but rather because our bodies are meant to be moving, not static.  So even though the rest of these tips are about how to set up your environment so it's ergonomic when you're sitting all day, the most important thing is to get up and move every once in a while even if you have a good set up. And even if you can't set up your environment so that it's ergonomic, as long as you keep moving around, you'll probably be pretty good!

2.  You should adjust your chair so that your feet can be planted on the floor and your knees are bent at a 90 degree angle.  If your chair isn't adjustable, you can also get a foot rest. If your normal chair is beyond redemption and your mind is boggling at how expensive chairs are, you can generally find some pretty good ones for around $20 on CraigsList!

3.  When you're using your keyboard or mouse, your shoulders should be relaxed, your wrists should be in the air (NOT resting on your keyboard or mouse, and NOT resting on a wrist rest), and your elbows should be bent at a comfortable angle.  The reason that your wrists should be in the air is that if your wrists are resting on something, then typing or moving the mouse will put more pressure on your wrist.
3a) If your desk is too high up for you to have your keyboard and mouse at the right level and it would be annoying to raise the desk, you can adjust your chair up and get a foot rest, or you can install a keyboard tray.
3b) Your keyboard should be roughly centered (meaning the center of your body should be between the "G" and "H" keys) so that you don't have to move your arms too far left or right when you're typing. Also, you shouldn't have to move your arm too far to use your mouse. That probably means using a narrow keyboard (e.g., no num pad, so it will be 87ish keys rather than 104ish keys) rather than one of the wide keyboards with all the bells and whistles. I like the Goldtouch Split Keyboard (not only does it not have a numpad, but the arrow keys and page up / etc keys are compacted into a smaller space too), but any narrow keyboard should work.
3c) For bonus points: our hands evolved to be in a handshake posture, NOT in a facing-down posture.  The Goldtouch keyboard I mentioned lets you use your keyboard in something close to a handshake posture. I also use an Evoluent vertical mouse for that reason.  Both of those are pretty expensive, and they can be hard to find used/cheap. I'm not aware of other vertical-ish keyboard brands, but there are a couple other mouses that approximate a handshake posture. A vertical mouse will also encourage you to use your shoulder and elbow more to move your mouse, which will get you to use your wrist less, which will reduce repetitive stress injuries.

4.  The top of your monitor should roughly be at eye level.  If your monitor isn't high enough, you can either get a monitor riser (they're pretty cheap) or just use a stack of books or whatnot.

5.  If you experience eye strain from using the computer all day, try to take a break every 20 minutes and look at something at least 20 feet away for 20 seconds.  Some folks recommend things to reduce the blue light, but my understanding is that that's mostly just marketing and that there isn't scientific support for it (humans evolved to be in sunlight, and sunlight has much more energy than a computer monitor, so it would be surprising if blue light from monitors caused eye strain). There is some evidence that "near work" (looking at a book or computer) is responsible for why people have worse eyesight now than they did in the past, so looking at something 20 feet away for 20 seconds every 20 minutes might also help you have better eyes in general.

6.  I'm bad about exercising, but whenever I'm feeling achy due to poor ergonomics, I do my yoga routine, and then I feel sore the next day and better afterwards.  If you don't have a routine that works for you, I mostly just do the Ashtanga Primary Series, and I'm sure you can find some resources for that online.