See Canvas Instructor Guide > Announcements for step-by-step instructions on creating and editing announcements.
You can also watch the following video tutorial:
IMPORTANT! If you are an instructor who has taught in Blackboard in the past, you are accustomed to automatically receiving an email when you post a course announcement. In Canvas, you must edit your notification preferences to indicate that you wish to receive an email of announcements that you have created; these are not sent by default. See Canvas Instructor Guide > How do I set my Canvas notification preferences? for instructions.