By default your announcement will go to the Classroom you are currently in. If you want to post your announcement in more than one Classroom, you can.
By default your announcement will go to all students currently enrolled in your Classroom. If you want to post your announcement to certain students only, you can.
Share with your class - write something
Topic - creating topics for your posts allows students to find them easier in your Classroom Stream
Add attachments...
When you're ready to post your announcement, you can either