In the upper right hand corner is the "Latest News" block. Here you can add new classroom information. Every time you add a new piece of news it can be emailed out to all students in that moodle class. Click on the "Latest News" link and it will take you to a text editor page. Type the message to your students, then scroll down to the bottom. If you want to just post it to your Moodle page click the "Post to forum" button. If you want to also email the news to your students then check the "Mail now" box before posting.
On your Moodle page the title of your new posting will appear in the Latest News block, along with the date it was posted. For your students to read the post they can either click the "more" link, or go to the "News Forum" link in the header block. The News Forum has all the news you have posted up to now.