Hey buddy, welcome and best of luck to you!
Typically, lesson plans involve coverage of specific grammar points...for example, "Can you (verb)?"
Rest assured, you can plan a 40min lesson that should cover some of these ideas:
Activity #1 (game of some sort)
Coverage of book (if given one)
Activity #2 (another game covering grammar point)
Most classrooms in Korea are equipped with up-to-date technology, so PPT games are super helpful across the ages.
For the younger ones, grammar should remain somewhat basic...introductions, I like I don't like, etc.
For the older ones, you can cover more advanced ideas : like opinions (I think you should), dream jobs ( I want to be____), etc