You can receive Here Comes the Bus notifications only if your school district is participating in the service. They will provide you with a school code to access the Here Comes the Bus website and app.
From there, you just need to sign up for an account, inputting the email address you want to receive the notifications. You'll start receiving alerts after that. See Creating an Account.
To receive push notifications on your mobile device, download and install the Here Comes the Bus app.