Chairs: Having the correct chair to support you properly while working is essential. The choices for good work chairs are almost unlimited. Prices range from quite reasonable to exorbitant. Since there are so many to choose from, and, since everyone is so different, do plenty of research and, whenever possible, try them before buying.
At the very least make sure the chair you choose supports your entire back and head. In addition the seat should be adjustable for height, depth and tilt.
Desks: Because sitting for extended periods of time can be very harmful for your back and neck, a great alternative to a regular desk is an adjustable sitting/standing desk (http://amzn.to/2wCiqwQ ). This type of desk allows you to stand or sit as often as you wish.
Recommendations for sitting range from no more than 10 minutes at a time to, at most, less than one hour at a time.
You can download a free timer from the Internet. These small apps pop up on your screen to remind you when the time you've chosen for sitting expires. Get up, move around, do a few exercises, get a drink of water, reset your timer and you're all set for another productive session of work.