Fix your chair height first
The single most important adjustment is free: set your chair so your feet rest flat on the floor and your knees sit near a right angle. If your chair cannot go low enough, a footrest — even a sturdy box — solves the problem instantly.
If your chair cannot go high enough, a firm cushion raises your sitting height. The goal is a neutral position where your thighs are roughly parallel to the floor and your elbows fall naturally to desk height.
Get the screen and keyboard right
Raise your screen so the top of it is at or slightly below eye level. A stack of books does the job perfectly for a laptop or monitor. If you use a laptop, an external keyboard and mouse are the cheapest ergonomic upgrades you can buy, because they let you position the screen and your hands independently.
Keep the keyboard close enough that your elbows stay near your sides and your wrists remain straight. The goal is to remove the reach and the hunch.
Movement beats furniture
No amount of ergonomic furniture can compensate for sitting still all day. Set a reminder to stand and move every 30 to 45 minutes. The best posture is your next posture — the simple act of changing position regularly prevents most of the stiffness and discomfort that chairs get blamed for.