The Zotero app is broken down into three panels. The first panel is your "Collections" -- these are kind of like folders or playlists. The middle panel are the individual records in your library, and the right-side panel is where you can view the metadata (the item details).
In Zotero, you can organize your library using folders, or collections. You can create as many collections as you want, and even create subfolders within those collections. This is really helpful if you are working on a specific topic.
If you are collaborating on a project, you can also create group libraries to share research. Click on the Group Libraries icon (just next to the Create a New Collection button), and choose "New Group." This will reroute you to the Zotero website, where you will need to log in with your Zotero account. Having a Zotero account is required in order to create or join groups in Zotero.
Once you are logged in, you'll need to name your group and choose the Group Type, which includes different privacy settings. Public memberships allow you to make your group library publicly accessible, and anyone can join. If you are working on a group project, use Private or Public Closed Membership -- this will allow you to share full-text PDFs (this is not allowed in Public Open groups).
Once you've created your group, you can then click on Member Settings, and click the Send More Invites button to invite others to your group.
If you see an dropdown carrot next to an item record, it means that there is an attachment.
You can add attachments by dragging and dropping files directly onto the record in your Zotero library, or by right-clicking on the record and selecting Add Attachement. You can attach any file type, including Powerpoints, PDFs, audio files, etc.
If you add a citation to your library but there is no full-text, you can always request it via Interlibrary Loan and add the PDF to the record later, once it is emailed to you.
Journal articles and books are generally imported correctly into Zotero, but there are occasional errors. Websites are less reliable, and you may need to add the website date updated, the author, or edit the website title. To do this, click on the record you want to edit. This will open up the metadata panel, where you can click into each field and manually edit it.