In upper right corner click Log in. If you are already logged in another account, pick circle representing your account and choose Add another account.
Fill in your login and password. You can find it in registration email.
Choose list of Google apps (square from 9 dots in upper right corner) and find Google Classroom.
Check that you have right account and finish account setting. Now you can see Classroom invitation which you can accept to access Classroom.
Your Classroom will be always accessible from Google Classroom and you use it to access new assignments and submit old ones.
