I'm 6 foot 3 and I have bad posture and I often get back pains. Who can help me?

Many things may. Physical therapy may help you work on your posture by increasing flexibility and strengthening your abdomen and back muscles. Regular aerobic exercise (getting your pulse over 100) can reduce back pain for many people.
Doctor. You want to have an evaluation by an orthopedic doctor or a chiropractor to find the cause of the problem.