You can get to a discussion in one of two ways: from the Discussions page on your course menu or through a link shared on a module.
If your professor has hidden the Discussions page, double-check your module for the week to see if there's anything with this icon: 
This icon indicates a link to a discussion.
To post on a discussion, click on the "Reply" button just below your professor's post. This is the initial post you'll make that responds directly to what the professor is asking. For example, you may need to read a blog post, watch a video, or reflect on this week's lecture content.
Some professors will set it up so that you cannot view anyone's posts until you make your initial post. If you don't see any other responses yet, don't worry—you're likely posting in the right place!
For more information on posting to a discussion, check out this on replying to a discussion.