There are two quick ways to access Google Classroom:
If using a Chromebook: log in to the Chromebook with your school student account. Then, open Chrome and go to Google Classroom on the web. You can type it in the web address bar, or click the shortcuts menu in the upper right-hand corner of the page. You will automatically be logged in and can jump to my classroom.
If using another computer: open up any internet browser and go to Google Classroom using the web address bar. When prompted to log in, log in with your school student account. You should then see any classrooms you joined. Find and click mine to go to it.
Click here for a more detailed and step-by-step guide.