Well, there are to my knowledge, 5 main schools of tai chi. I went to a Yang style school. Have never heard of the one Hops mentioned. Doesn't mean it's not a valid school - it just isn't included in the background learning I had to do for ranking. The five we were taught were: Chen, Yang, Wu, Wu Hao, and Sun.
A good instructor should ask off the bat why you're interested in tai chi, what you're hoping to gain from it or your interest level. That's the time to bring up physical limitations and ask if they're willing to work with/around yours. They should have an established location for classes and hopefully, a lengthy amount of experience. But new teachers are approved each year, so that last one probably isn't as important as being able to communicate easily and on the same level as your instructor. Talk to fellow students in your first sessions. Note that the study of the long form doesn't necessarily follow a calender year. In our school, section 1 training begins in September. Day and evening sessions were offered at my school. Beginner's classes may form any time of year.
In the first sessions the simplest chi gung and basic principles are taught. As well as proprieception - being aware of being in your body and how that feels; learning to be in control of the basics of moving & breathing. Yeah, breathing plays an important role in this - and contributes to building strength and stamina. But DIFFERENTLY than yoga.
Does that help answer most of your question? It really comes down to your relationship with the teacher and what your goals are. And that's all personal stuff.
