You won't find a more comprehensive guide than this from Kelly Fitzgerald. While intimidating at over 80 slides, flick through to the ones you want.
More resources to explore
Copy of Google Classroom - tic tac toe of activities
Additional how to's for features of Google Classroom
Originality reports - this guide and video tutorial by Eric Curts gives a succint overview and how to use for easily checking 'copy and paste' thinking.