The very first thing you need to do is go to Google Classroom. Now you need to create your first class. All you need to fill out is the class name, the rest you can leave blank.
Click your class, then at the top, click on Classwork. Now you can create your first assignment, quiz, question, or material.
Now you just have to add your child into your class. You can invite them by email, or you can have them join the class with the class code that appears in the classroom header.