If you have not set up syncronization before, follow the steps in the First Time Synchronizer box on this page. You only need to do this once.
After setting up synchronization the first time, there are two ways to keep your data synchronized:
Synchronizing is how you keep your data up-to-date on both your local machine and on your Zotero cloud storage at the same time. It is very strongly recommended (and nearly mandatory) for Zotero users.
These are the steps for setting up syncing for the first time. Once you've done these steps, you won't have to do them again: