When you log in, you will be taken to the Dashboard, which shows 3 ways to interact with the WCM:
If you manage sites in different divisions, or a personal site, this dropdown lets you switch between them. Each site has a base, like www.csus.edu/irt or www.al.csus.edu that is represented by the Base Folder in the Asset Tree.
The menu bar is the primary way to create new content. The "New" menu has predefined templates to create new pages for your website. It is also how you upload files or create new folders.
The Asset Tree is a familiar tool for getting around a website when you're building it. Like Windows Explorer or Dreamweaver, files are organized into folders, which can be expanded or contracted.