After you have created your site and added some content to your homepage (instructions), you can add additional pages to your site.
Create a page. So, go to Sites.Google.com, and you should see a list of the sites you have created. Click on the link to the site to which you want to add a page. Then, when you see the homepage of the site, click on the Create page button in the upper right-hand corner of the screen.
Template and location. In addition to the basic webpage (which is probably what you want to choose), GoogleSites offers some other kinds of pages that you can build. In addition, you can also choose what kind of address you want. By default, GoogleSites will create a webpage in the same directory as your homepage ("Put page at the top level"). Unless you have a specific reason for changing this option, just leave it the way it is.
Page name. You can type in the page name with upper- and lower-case letters, spaces, and so on, and Google will adjust the page name so that it will function as a web address (all lower-case letters, no spaces). You can force the address to be something else, but you will probably just want to use the address that Google suggests. When you are done, click on the Create Page button at the bottom left-hand corner of the screen.
Edit and save your new page. You can now edit your page following the same instructions as for editing the homepage - adding text, links, images, etc. Make sure to use the Save button in the upper right-hand corner of the screen to save your work when you are done!
Site navigation: It's automatic! Notice that your new page is automatically added to the navigation sidebar on your homepage! That's a big advantage of the GoogleSites editor: you can go in and make changes if you want, but by default it builds the simple site navigation for you!