When you visit a page that Zotero recognizes as a bibliographic item, such as an item in a library database, an icon will appear in your browser's toolbar. The appearance of the icon depends on the type of resource being viewed, with the most common being the book icon and the lined page icon representing a journal article. Some of these items are very difficult to see.
If you're on a web page that Zotero can't identify as a particular type, you'll see a generic page icon. Clicking on this will add a basic entry (often just the URL and the date accessed) for the page to your Zotero library, along with a snapshot of the page.
When you click on the icon, information about the item you are viewing will be automatically entered into your Zotero library.
If you need to add an item for which you have no online record, you can add the item manually using the "new item" button (the green circle with a plus sign), and choosing the type of resource that you want to add.
Once you have selected the appropriate document type, details about the document may be entered in the right-hand panel.
When you save an item to Zotero, look at the data that is being saved. Zotero may have missed something that you need (like the abstract) or recorded data incorrectly (e.g., forenames and surnames mixed up, title in all caps). It's easier to fix mistakes like this when you're collecting the data that it is to go back later when you're trying to put together your bibliography.