Imagine having OneNote full of pages of notes, and you can't find anything. What would you do?
Fear not, OneNote has you covered. There's a powerful and quick search feature in OneNote, which means that you'll be able to find things when you need them. To search, just click in the Search All Notebooks box, and then enter your search term or keyword.
But when your notes become really numerous, you might decide that having an organized structure (or filing system) is a good thing. Notebooks and sections are available for just such organizational purposes.