Proper posture while seated should include your feet flat on the ground, directly underneath the knees. The legs form right angles with your butt on the chair and your knees coming out directly straight from your hips. You want to avoid having your legs dangling from a chair that's too tall or sitting in a cramped style.
Moving up the body, focus on the spine being in proper alignment. Often the spine rounds forward as you're typing on your computer or working on something in front of you. Fix this by bringing the abs in toward the spine, lifting the chest and rolling back the shoulders. You want your shoulders to be stacked over your ribs, and your ribs to be stacked over your hips.
Next, bring back your head so that your chin is over your sternum or your breast bone — not jutting forward over your body.
Finally, rest your arms on a desk or surface that provides a 90-degree angle so that your elbows can be bent at your sides and your forearms stretched out in front of you onto your computer or surface.
Ideally, you'd have your computer monitor at eye height and your elbows hugged in at the sides of your waist. However, if you're working on a laptop computer, that is near impossible without an external keyboard. So, focus on the elbows hugging in at your sides with your forearms extended out to the keyboard, and look down to the computer screen with your eyes while keeping your spine in alignment and your chin back.