One of the earliest recorded teachings in the Buddhist canon is something called the Sutta-nipata a collection of discourses by the Buddha to various disciples. These are not sutras in the strictest sense, but are part of the earliest collections, and represent some of the closest things we have to the Buddha’s original teachings.
In the Simile of the Boat, verses 316-323, the Buddha talks about the importance of good, reliable teachers:
“He from whom a person learns the Dhamma should be venerated, as the devas [the gods] venerate Indra, their Lord. He, (a teacher) of great learning, thus venerated, will explain the Dhamma, being well-disposed towards one. Having paid attention and considered it, a wise man, practicing according to Dhamma, becomes learned, intelligent and accomplished by associating himself diligently with such a teacher.
Translation by John D. Ireland
The importance of a good teacher in Buddhism is to help clarify and explain the Dharma, the teachings of the Buddha, in a way that you can learn and apply in your own life (and thereby benefit from it). The teacher holds no power over you (nor should they), but they are responsible for teaching the Dharma properly. The Buddha described this as catching a poisonous snake properly.
If a teacher can’t do it properly, not only can they be “bitten”, but their students who received the bad advice, get bitten too:
“But by following an inferior and foolish teacher who has not gained (fine) understanding of the Dhamma and is envious of others, one will approach death without comprehending the Dhamma and unrelieved of doubt.
Translation by John D. Ireland
As a student, one has to ask: how do I find a good teacher, especially in the West where teachers are few and far between? As Brad Warner writes, there are plenty of people who enjoy the allure of being a teacher. Such teachers may start with good intentions, but go astray, and lead plenty of people with them.1
“Besides, I have never encountered anyone who started out wanting real bad to be a dharma teacher and then ended up being a good dharma teacher. Their desire to be seen as A Dharma Teacher always overshadowed whatever they were supposedly teaching. It was more about performance than substance.”
brad warner
And so the most important thing is substance: a genuine commitment to the path, experience, and sincere desire to help others rather than their own image:
“But if (the man at the river) knows the method and is skilled and wise, by boarding a strong boat equipped with oars and a rudder, he can, with its help, set others across. Even so, he who is experienced and has a well-trained mind, who is learned and dependable, clearly knowing, he can help others to understand who are willing to listen and ready to receive.
Translation by John D. Ireland
Such a teacher is worth placing your trust in.
1 This is a big reason why I quit training in 2017 to be a minister with the Buddhist Churches of America, and with Jodo Shinshu Buddhism in general. In spite of my personal misgivings about that particular sect’s teachings, the allure of being an ordained minister had clouded my judgment and made me compromise my beliefs in order to keep going. In the end, I realized my error, and decided it was better to be a sincere lay person than an insincere priest faking it for an audience. I lost a lot of blog readers, and friends at the temple, but in the end I am a lot happier with myself now, even if greatly diminished.
Gleanings in Buddha-Fields 
