In Zotero, go to your Preferencesmenu (on Mac, Zotero > Preferences; on PC, Edit > Preferences)..
Under Data Syncing, enter your username and password.
Check the Sync Automatically checkbox.
If you would like PDFs and other downloaded files to be available from your online account, next to Sync full-text content, check the checkbox. Please note that you may wish to change this setting or upgrade your account if you reach the storage limit for free accounts.