When you log in using your Google account, you will automatically be connected to your teacher's Google Classroom classes and you will receive any assignments that they post.
If you're not convinced that you are connected to your teachers' classes, it could be due to the following reasons:
You did not connect using the Google account that you used to connect to Google Classroom.
Your teacher does not use Google Classroom.
Your teacher has not posted any assignments in Google Classroom recently.