You can organize your library by creating folders, which are called collections in Zotero. You might find it helpful to organize items by theme/topic or by a specific research project you are working on. You can create a new collection by clicking on the "New Collection" icon or right-clicking on "My Library" and clicking on "New Collection."

You can also create subfolders by right-clicking on a collection and clicking on "New Subcollection." Right-clicking on a collection will also give you the option to rename or delete a collection.