Frankly Maddie.. I would go in to your Tresor Publique asap and get this underway.
Go armed with the one-page Certificate the Notaire will have given you... Which gives details of who has sold and who has bought...a certain property. (You will have needed it to sort out Electricity/phone/water etc etc)..
and take your passport too..
Make sure the Tresor Publique have the correct address to which Bills should be sent (if this is different to the Property address).
They will be able to explain what docs you need/how to get Prelevement Automatique organised.