Hello! I'm creating a serie that's triggered from an event, but this event already exists for a while, how can i start this serie with only new events?

Just above, Triggers > "When to send" > "+ Add event or data rule". Set the event there. You will not have the "greater than" option as it should trigger whenever the event happens, and that is when the user will go into Series.

Also, mind the option on the top - for re-entering the Series. If you use events as I mentioned above, and they happen multiple times, the user might re-enter the series if that option is ON.