a good teacher is essential
In anything that you choose to pursue, it is of course advantageous to have a good teacher. If you are going to commit your time, energy, and money, you want to be getting good value for all of those.
In learning Taijiquan in particular, having a good teacher is of paramount importance. It is virtually impossible to go beyond the absolute basics without hands on correction from an experienced and skilled teacher. This in largely because the process of learning Taijiquan is one of bringing the unconscious into the conscious. And since our unconscious is just that, doing this alone or without expert guidance is like groping in the dark.
difficulties as a beginner
But as a beginner, it can be difficult to know who might be a good teacher. At present Taijiquan has no industry standard, meaning virtually anybody can do a short teaching course and start to teach. For a beginner it will be very difficult to discern a bad, average or good teacher. And to be frank, there is a large amount of rubbish being taught in the name of Taijiquan, that amounts to little more than 'arm waving'. This could result in getting discouraged and being put off all of Taijiquan, which would be a great shame as authentic, good quality Taijiquan is a beautiful and valuable thing.
the importance of lineage
There was also no industry standard in ancient China. This is why lineage is so important and has traditionally been the way of assessing someone's credentials: because it gives some indication of whether someone has had access to authentic Taijiquan.
That is not to say that anyone in a good lineage will be a good teacher or even practitioner for that matter. There are of course many elements, including length of study, dedication, contact time with the teacher, and innate talent. But to put it the other way around, you can be pretty sure that a person that does not come from a strong lineage will not be a skilled practitioner, and so teaching isn't even a question. It is the blind leading the blind.
So the first point is go to someone from a good lineage. After that you can start to use your own discernment, about the level of teaching and also if the style of teaching works for you. My feeling is most people will instinctively be able to recognise good quality Taijiquan. There should be a grace to the movement, yet at the same time a feeling of power. It should captivate you, at least for a moment.
things to look out for
hands on correction
Hands on correction of postures is absolutely essential to learning Taijiquan. If a teacher isn't physically correcting your postures, it is a very bad sign.
lots of forms
Learning the external choreography of a form is only the first step in learning Taijiquan. It is at the pont where you know the external choreography that you can really start to develop. So if a teacher is always throwing new and different forms at you, or advertises their credentials by the number of forms they know, this may be a sign that their knowledge of the subtler aspects is lacking.
Applications of the movements of Taijiquan is something that it's only really necessary to know once you have developed the internal biomechanical principles to a high level.
Learning Taijiquan is a primarily experiential process. If someone is spending half the lesson talking or explaining things, this is again a bad sign as to their real level of skill. Understanding intellectually is meaningless if you have not developed the same understanding in the body.
Grandmaster Chen Zheng-Lei
Grandmater Wang Hai-Jun