If is out of adjustment, adjust it. If it squeaks, oil it. If the casters do not roll, free them up or replace them. Go no father until you are happy with your chair. Do not compromise on your chair.
Bad chairs are half the problem.
Place your good chair in the middle of the room away from the desk and sit in it. Use good posture but not ram-rod posture. Try to sit as you sit to type but do not slouch. Adjust the height of the chair so you can place you feet lightly on the ground. Your calves should be sloping down slightly toward the front.