I never had back pain ever, but just after a SINGLE day sitting in my new workplace's chair, my back was in pain.

I think the way you sit is paramount, so I'll just describe what works for me: Remove the arm rests of your chair. This allows you to get the chair close to the desk until your belly touches the edge. The table itself will support your arms.
I found that this takes a lot of pressure out of your back. Besides that, "untuck your tail", as in, slightly arch your lower back projecting your butt out. You can also lean back a bit as that distributes your weight better in the chair (though I change positions often).