If you are doing 5 minutes, 10 minutes, 15 minutes sessions every day you don't need teacher. It’s a simple mental exercise. You may feel no effect on your mind to caffeine kick kind of effect.

But if you are doing more 30 minutes per day or specially more than one hour, you need some one guid you.

Best teachers are people spending more time in meditation than you are.

How to find these people are different from region to region.

I’m Theravada Buddhist and we have some monks living in jungles or border of jungles, they are full time meditators, wake up early -> meditation -> breakfast -> meditation -> lunch -> meditation(no dinner) -> sleep. we usually go to these monks for instructions when doing series meditation.