big part of feeling comfortable while sitting down part of feeling at ease is your choice of furniture. A well-designed, ergonomic chair that provides support can correct your posture and prevent the body from leaning forward and slumping. Your desk must also be suited to your height. You must be able to comfortably rest your elbows at your sides and not put stress to your wrists.
It can also be useful to add functional accessories to your workplace. If the desk you have is made to accommodate taller individuals are, then you could benefit by adding feet rests. Also, it is helpful to include work lights to make your computer and desk easier to see without any strain on your eyes.
Make sure that your monitor does not cause discomfort. Your screen must be located at or just below eye level and about 1 meter away from your face. Of course, the exact position that makes you feel the most comfortable might differ according to the person you are and so it’s a good idea to experiment to figure out which position feels most comfortable for you.