To add a new page, if you have widespread editing privileges you can go to "Create content" at the top of the right hand menu. If you don't, you need to ask the webmaster to do the following steps to create a blank page for you, with a name for the menu.
Click on "Page"
Give it a name - shortish (it will be used in the menu bar to the left
Select a 'parent' page - this must be one of the pages you are allowed to edit. This makes the page appear as a daughter of your original page (or grand-daughter, etc).