If you have poor posture, then your pelvis is probably not in a healthy neutral position. You might find it helpful to have some sort of professional assess your posture and find out whether you have an anterior or posterior pelvis tilt. This is really, really important to know for sure, because if you have an anterior pelvic tilt and you're doing corrective exercises that are more appropriate for a posterior pelvic tilt (or vice versa), you will actually end up making your posture worse. It may be pretty obvious though which position your pelvis is in if you do some Google research on anterior and posterior pelvic tilts, so start with that and keep the idea of a professional posture assessment in mind.
For the upper body, the common problems are forward head posture, forward rolling shoulders, and a variety of issues with the shoulder blades. If you have problems with all of those areas, you probably have upper cross syndrome. Again, a professional posture assessment will help you figure out exactly what needs to be done to fix your specific problems, but you can also learn a lot from Google research.
Moral of the story: there is no one-size-fits-all approach for fixing posture. You have to figure out exactly what issues YOU have before you can fix them.
