When you sign up for Piazza, make sure you are signed up for the correct term (Fall 2017) and lecture (B lecture) of CS 173. Ignore any warnings from the Piazza sign-up about requiring a U. Illinois email address: you should be able to register using any email address you like, including an anonymous email id.
Very little appears directly on this page. Routine announcements are posted on Piazza, Moodle, or the notes from each lecture. Check out the menu above for the Lectures page and information on other topics such as course policies, homeworks, and exams.