Really don't want to be super short and hunch backed when I am older. How to change my posture?

See spine doc. If your back is not straight now and/or if you have a family history of such changes with aging, then go see a good primary physician. If there is any unusual curvature of the spine, he/she should refer you for xrays & further eval with specialist. Be sure that your bone health is good (many people need extra calcium, etc.) and stay fit.