Creating a free Zotero account
Visit www.zotero.org/user/register, choose your desired username, provide your email address, and designate a password.
Try not to immediately forget your username and password -- you'll be using them again very soon to sync your Zotero desktop library to your Zotero account (more on the benefits of this below).
Once your account is created, you'll be able configure and fill in your profile, but you don't have to do this if you don't want to.
It is important to sync your Zotero library to your account.
Why?
Syncing transfers folders, library citations, notes, links, tags, attached PDFs, etc. -- everything that's in your Zotero library.
To sync your account
1. Under the Edit menu in your desktop library (this is the Zotero menu if you're on a Mac), choose Preferences.
2. Choose "Sync" (the second tab in the Preferences window), and enter your Zotero username and password when prompted.
3. Your library should start syncing automatically once you click "OK". To force your library to sync, click the small green arrow/circle icon at the upper right of your desktop library above the Info pane.
Zotero's website not only enables you to view your Zotero Library, but it also lets you create, manage and edit items by using the editing icons above your list of resources or by clicking on an individual item. As always, you can view any attached items through the web interface.
Editing an item in your Zotero library: