You should have an AnkiWeb account to be able to synchronize your cards. The cards won’t be available on AnkiWeb otherwise.
If you already have an account, then you can sign in by clicking the sync button on the main window of the desktop app and synchronize your cards.
After signing in, you may be asked whether you want to upload the empty desktop collection and override your collection on AnkiWeb, or download the collection on AnkiWeb. You should choose download in your case.
Note: I recommend watching the videos linked above at least to have a better understanding and to avoid data loss.