You can use the left column of the Zotero Library to organize your citations into different folders and subfolders.
1. In the top left corner of the left column, click the icon. A window will appear, asking you to give a name to the folder. Enter a name and click OK. This folder should now appear in the left column of your Zotero Library.
2. To create a subfolder, right click on the folder you would like to add a subfolder to and select "New Subcollection". Enter a name for this subcollection and click OK.
3. Add references to your folders by dragging and dropping items from the center column into the created folder. If you highlight a folder in the left column, items you add to your Zotero Library will go directly into that folder.