As you're starting to use OneNote the first thing to consider is notebooks. When you use OneNote, your notebooks are stored in the cloud. (Why does that matter? You can access your notebooks in OneNote from ANYWHERE. Phone, work computer, home computer, tablet. THIS is THE MAIN reason OneNote is as powerful and useful as it is.)
To determine how many notebooks you should setup, consider 3 things.
In general, my advice is to start with fewer notebooks. You can always organize by sections and pages within a notebook. Although organization might be a reason for more notebooks, the primary reason is sharing.
If you plan to keep only one type of information, for example personal and you aren't sharing it - you only need one notebook (at least to start).
If you, like so many who use OneNote for work, plan to share information consistently with a group of people, that will likely be it's own notebook. Most common example, a group of people on a committee to share meeting notes for a board, project, team or department.
If in doubt, start with 1 notebook. You can easily add more and move things around.
A note about accounts. Many of us have a personal account, a work account and sometimes more than that. Keep in mind you want to be aware of which accounts you are using. This probably makes up most of my questions about getting things setup in OneNote. Users have more than one account and they are signed in with different accounts on different devices and can't see the same things, therefore missing out on the biggest win with OneNote, redundancy across all your devices.
To see how you're signed in, click the picture or initials in the upper right hand corner of your OneNote app.
If you want to add a notebook, click on your notebook and click + Add Notebook.
Decide where to save your notebook. Keep in mind, the benefits are to save it in your company OneDrive if you want to share it with other users in your company. If you are creating a notebook just for you, be sure to save it in the cloud so you can access it from any of your devices. If possible, ALWAYS save to the cloud associated with your sign in account.
Name the notebook something obvious so you can find it and know what it contains easily, PERSONAL, Board of Directors, Finance Team etc.
Share your notebook by clicking File...Share... and either enter in names of internal users or copy the sharing link and paste it in an email or Teams chat or anywhere else.
Notice when you share you can give view or edit access.
Switch between notebooks by clicking the Notebook name and selecting another notebook from the list of notebooks.