With the online account you are able to start building a Zotero citation dataset on the website. These references can be synced (shared) with other computers and word processors (MS Word). The online account also provides downloads for the desktop software and access to online tutorials. The Documentation tab opens a list of tutorials and documentation to assist with navigating Zotero (https://www.zotero.org/support/).
Once logged into the webpage, click on "My Library" to view the references currently in the library. This interface can be used anywhere to enter references into the library for future use. The website allows information to be entered, but does NOT directly interface with the word processing software. You will need the desktop application in Step 3 to use Zotero with MS Word.
View the video below on how to enter new references into the webpage library.
https://youtu.be/WNFCoI6pWYM