Registering for a free Zotero account gives you a number of benefits, including the ability to sync your library between computers and the Zotero cloud, participate on the Zotero forums, and create and participate in shared Zotero groups. To create an account:
When you have completed creating your Zotero account, see Syncing Your Library to sync your source data to zotero.org.
In order to back up your citations to the Zotero cloud and sync your library between computers, you'll need to set up Zotero Sync with your Zotero.org account.
To set up Sync:
If you use Zotero on more than one computer and want them to stay in sync, repeat this process on each of your computers. Any updates you make on one of your computers will be reflected on the others. This even works to synchronize your library among Windows, Mac and Linux computers. More about Zotero sync