You have to strengthen your muscles to have better posture. The old trick of walking around with a book on your head can actually help. When you are sitting on a bench, couch, chair etc… don’t lean up against the backing. Sit, not slouched, with your back straight. It’ll help build those muscles. Simple yoga can help as well. To know if you have good posture photo yourself standing and sitting as you would normally (no posing). Better yet video tape yourself. Many many people think their posture is good before they see a video. In regards to the low back pain, i’d have it checked out by a doctor. Sometimes there are physical causes that need surgery. If there isn’t one then you can explore alternatives like chiropractics, acupuncture, massage, yoga, etc…