Zotero Bookmarklet: Allows you to save items from mobile devices and other browsers.
Zotfile: Bring advanced PDF management to Zotero with the optional Zotfile add-on.
More Zotero plugins are available here.
Register for an account
When you visit Zotero.org and click on "Register," the website will prompt you to create an account. Registering for an account allows you to sync your Zotero library to the cloud, to create group libraries to collaborate with other Zotero users, and to monitor your storage usage.
Sync your Zotero library with your online account
Once you have registered for an account and verified your account by clicking on the link in your email, you should sync your Zotero software with your online account. Open your Zotero program, then find the gear icon in the middle of the pane and select "Preferences" from the drop-down menu.
Under the Sync tab, enter your username and password.
This will sync your account. Now any time you add something to Zotero on your computer, it will also be added to your online Zotero cloud account automatically.