You start from the hips and work your way up. Sometimes you have to start from the feet if people have foot issues (flat foot etc). You want decent flexibility in your hamstrings and hip flexors a strong set of glutes and abs, strong rhomboids, mid/lower traps and rotator cuff and strong neck extensors (not overly stretched).
For this you need a proper postural assessment (just a visual look at you in boardies by someone who knows what they are doing) and then you go from there. It is a lot of work and something I can not do properly over the net. But to get you started get flexibility in your hips, keep lifting as you are, add in some good core work (3 sets of barbell rollouts would do at the end of a workout), put in some scapula retractor and depressor work to help pull back and strengthen those shoulders (3 sets at the end of a workout of band pull aparts or reverse dumbbell flyes are fine). Most of all try to use a good posture throughout the day (sitting and standing).