Getting Started

Step by step website (by Trevor Takayama)
Step by step guide (searchable)

Adding Content

Set up a course (by subject or class period)
Managing courses (resource page, permissions, inviting students)
Adding Subgroups (Units, subjects, groups or topics)
Assigning work to whole class or individual students

Things to Consider

Google Classroom Tips and Tricks (by Steve Wick)

More Help

Google Classroom cheat sheet
Alice Keeler's extensive tutorials 
Even more help