Each notebook is a single tree of notes.
 
When you create a notebook, you create a new tree of notes, starting from the root (invisible) note.
 
You can create new notebook from the left panel:
new-notebook-button.png
 
There are plenty of notebook management options you can access with its menu - just click right to the notebook name:
notebook-menu-recent.png
 
You can learn more about these options by clicking on the "managing notebooks" tag above.