In Zotero you have your full library of citations -- and then to use Zotero for multiple projects and organize your citations, you may want to create particular "collections" of citations.
Once you've imported several citations that you want to organize into a collection, your first step will be to create a folder using the icon on the left side of the page:
Then select the citations you want to move into your new collection, and choose the "Add to Collection" button:
Once you've moved items to a collection, click the collection's title to see those items alone rather than your full library. One item can be in multiple collections at once, and you can even create collections within collections (for example, if you're doing multiple assignments for a class):