the use of the article 'a' means the speaker knows about the lesson, but the listener does not. Thus, a teacher would never use it to describe a planned activity. A real teaching lesson is always a planned activity. The teacher would say 'Let us now begin our lesson' or 'It is time for the lesson'. so the literal meaning of this phrase is 'I just realised that you do not know this, and I will stop what we are doing to teach you.'. Because it is not spoken by a teacher, this phrase happens outside of a Teacher / Student interaction. The person speaking the phrase is assuming the role of teacher, without permission. This is a bold, authoritative action. Usually it is stated much more aggressively, and figuratively. 'I will teach YOU a lesson!' is a really strong expression. The speaker will not actually teach a lesson, they are really saying 'I am about to do something, in response to your actions or words, that will cause you to regret your actions or words. You will remember my action in the future, and not do that again!'

someone could say ill give you a lesson, or ill teach you.....but if someone you have a problem with ever says "im gonna teach you a lesson" I suggest you stay away from them.

No, I'm pretty sure it is never used like that.

If someone says they're going to teach you a lesson... run! :)