One of the basic ideas of OneNote is to keep "all your notes in one place" so
all of them are available as soon as you boot the app. The organizational
units within the app are: folders, sections, and pages/subpages. Folders
contains sections and sections contain pages/subpages. Tabs that run at the
top of the screen are sections, tabs that run vertically are pages.
Usually people create a section per project/topic, then add pages as the
project develops. E.g. I have "Math for kids" section and add a new page per
each new interesting problem I find, then I add subpages with comments on how
the problem faired with different kids. You can also check Helpful Tips
section (if you are on SP1) to see an example of a section with a few pages.