You may have received an email from your teacher inviting you to a Google Classroom. Clicking on the Class link in that email will add you to that class.

However, if you have been given a Class Code by a teacher, use these instructions:

Go to https://classroom.google.com

Click on the plus (+) symbol in the top right corner

Click on "Join class"

This will open a pop-up window in the center of the screen

Enter in the previously shared Class Code

Click on "Join" in the bottom right