Flat back syndrome

I have a flat back (loss of curves in the spine). And also my shoulders loop a little forward. I look weird from the side, and my stomach kind of sticks out while my ass goes the other way.

Kind of like the third from the left on this picture: Viderekoblingsmerknad

I have some pain in my lower back, shoulders and neck. I am very stiff in all my muscles. What muscles should i focuse on strengthening and what should i focuse on stretching to improve my posture and get more pain free? What weight exercises should i avoid, and which should i emphasize?