Just type the name of the new page. Many Web-based wikis insist that you use something called CamelCase for page titles that have multiple words -- for example, BrighthubTopics. You can do that in Zim, but it's not necessary. For non-CamelCase titles, highlight the title and press CTRL-L to make it a link. Then, click the link. A new, blank page appears.

click to enlarge
On the new page, start typing whatever it is you want to type. When you're done, choose
File > Save. You can navigate back to the main page, or any other pages, by clicking an item in the menu on the left of Zim's window. Or, you can use the buttons at the top of the window to move between pages.
In part 2 of this article, I'll look at some of Zim's other features and explain at how to format text.