There are many ways to add new items to Zotero, but the easiest is to use the Zotero connector. Zotero will recognize information about books, journals, and other items on web pages as you search for sources. It will display icons depending on the kinds of sources it sees, which you can use to add entries to your Zotero library.
For example, if you are viewing a page with information about a book, like an entry in a library catalog, Zotero will display a book icon. When you click on the icon, Zotero will take information from the web page and create an entry for that book in your library.
Zotero recognizes many kinds of items, and will change icons accordingly.
In each case, Zotero will create an item in your library with the information you will need to cite it later. In some cases, Zotero will do a little more:
If you save to Zotero from a page about a journal article, whether in a database or on a publisher's web site, Zotero will try to retrieve the PDF full text and store it with the item. (If it doesn't find the PDF, you can add it to your library manually by downloading the PDF yourself and dragging it over the item in your library.)
If you are viewing a page of search results in a database or on Google Scholar, Zotero will show a folder icon, which will open a window with a list of the results. You can add several items at once by selecting the titles you want and then choosing "OK."
If Zotero doesn't see information about a source on the page you are viewing, it will offer to store the information you will need to cite the page itself, including the date retrieved. Zotero also stores a snapshot of the page at the time it was saved, which can be very useful for news sites or others that update frequently.
If you have a PDF of an article, you can add it to Zotero just by dragging it over an empty spot in the Zotero window. Be sure to check the results, since this method depends on the PDF having properly formatted data. If the information doesn't look right, add the item using some other method. You can then add the PDF to your library by dragging it over the new item.
You can add items to Zotero using identifiers such as ISBNs, DOIs, and PubMed IDs. Choose the "Add Item(s) by identifier" button from the Zotero toolbar and enter the identifier. You can paste several identifiers, each on a separate line, to look up several items at once. Zotero looks up the item information and creates a record for each.
You can always enter an item manually, of course. Choose "New item" from the Zotero toolbar, and select the item type. (Less common types can be found on the "More" submenu.) Then enter the item information in the appropriate fields.
If you're an EndNote user, you can export items from EndNote and import them into Zotero. The basic process is:
For full details, see: https://www.zotero.org/support/kb/endnote_import