
Wu JianQuan,
founder of Wu Style
If you were going to buy a new car, you'd probably check out a few models, test drive them and find one that fits your price range and requirements. Learning T'ai Chi should be approached in the same way (unless there's only one "car yard" in town).
If you're a complete novice: check out all the groups around. They should give you a lesson for free. Do it, don't just watch. Feel it. Do you like the teacher? Are the class times convienent for you? Can you get private classes (if you wanted to)? What credentials does the teacher have? What do you get for your money?

Ma YueLiang, 1994
I cannot stress how important the lineage of a Taijiquan teacher is. No matter which style you learn, it is important to learn from teachers with a lineage of recognized masters. It is only by learning from teachers with a strong lineage of recognized masters that students can obtain the full benefits of this amazing health system that has been carefully passed down for centuries.
A teacher should not be afraid to authenticate their credentials. If there's something in Chinese, ask for a translation. Check them out thoroughly. Ask them three questions:

Yuan ShunChuan, 2002
Once you decide on a group, stick with it until you complete the first course (at least six months) and see how you feel after that.