Login to ReachMyTeach with your school email. You can select Sign in with GMail or with Outlook if you use those providers. Student, teacher, and guardian contact information will already be connected through your school/school's SIS, so you should be able to login using your school credentials.
We encourage you to do the walkthrough that will help you understand how to send your first message. If you would like an overview of how to send your first message, what it looks like, click here.
More walkthroughs here.