By default, when students open your course, they will be on the Announcements page. However, sometimes this gets changed (deliberately or not) and you need to change it back. In order to do that, you'll need to change the Course Entry page.
Here's how to change the Course Entry page:
1. Under Course Management on the Course Menu, use the chevron on the left to expand Customization, and choose Teaching Style.
2. Under Select Course Entry Point, choose Announcements.
Then hit Submit at the lower right and you'll be all set.