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.
You can add notes and tags to items in your Zotero library. There are tabs for notes and tags in the right-hand pane next to the "Info" tab.
Tags provide another way for you to organize your library. You can search by tags using the "All fields & tags" search box. Some databases will automatically provide tags (or keywords) when you add an item to your library. You can disable automatic tagging by going into your Zotero preferences.