The main sections of the site are shown in the blue bar across the top of every page.
Click on any of these to go to the relevant section. Some of the items are drop-down menus containing sub-items. Just move the mouse over the bar to see the list, then click on your chosen item.
See also The Home page.
There is a secondary menu in the side panel on the right, this takes you to different types of content (Articles and Galleries), plus the Add content link enables you to do just that. See Contributing for more details of what you can do.
Once logged in, this appears below the Navigation menu, and contains things relating to your account, as well as a link to this guide.