With tai chi, you should start learning posture corrections from day one. Posture is so important, not just to look good, but because it affects all  your internal organs and your spinal health.

I see so many people walking around with terrible posture -
         shoulder slumped to one side, while the other is too high
         top half of body bent forward
         neck bent down, so the person is looking down
         butt stuck out
         high heels

All of these will cause problems sooner or later, quite needlessly. Tai chi will teach you to correct your own posture, to know what is wrong with it and how to realign your body and spine correctly. There is nothing more important for your health than correct posture. After I'd been doing tai chi for a while, I could see there were various things wrong with my own posture - shoulders too high, vulture neck, hips up to one side. Without doing tai chi, I would have remained blissfully unaware of these, but now they have been corrected - not only do I feel much healthier, I also walk like a queen! Does wonders for self-confidence....

Just doing tai chi every day will help you become much more aware of your body, which will help you recognise when things are out of kilter, so you can ask for advice on how to get yourself sorted out. You will be in harmony with your body, instead of taken by surprise by its failings after years/decades of poor posture.

If you're into martial arts at all, then body posture becomes absolutely crucial in executing any application, since without it you can't get your body weight into the move, so having to rely on crude muscle strength instead.

Anyway, learning how to check and correct your posture is one of the first things you should be doing in your tai chi lessons, so make sure you ask your instructor about this. Enjoy your posture and how it helps your good health!