If you’re going to be spending hours and hours in your office, invest in the right furniture as well. Kitchen chairs don’t typically offer a lot of back support and can cause fatigue after just an hour or so.
Look for an office chair that is fully adjustable – height, back, armrest, everything. A good position for someone else is not going to automatically work for you. Getting a chair on wheels is also good, since you probably need to get up from your desk several times per day.
If you need to do work at your desk other than just be at the computer – taking order over the phone, writing things out, and so on – you need a desk that’s big enough to accommodate. A small computer cart won’t be enough.