Some people gradually get their balance on the exercise ball that you can sit on. It has to be an exercise ball tall enough so you can work at the desk with your shoulders in a relaxed position.

For head posture, just imagine a string attached to the top of your head that pulls your head up in line with your spine.

There is another tip to help posture is to tilt your pelvis forward. This should help your spine go into a more natural position. (A long while back I used to teach exercise classes.)