If you have a checkbook, you'll find « RIB » are at the end of it. If you have no checkbook, ask one at your bank.
Go to your bank with the details of your bank account : a « relevé de compte » is ok, your credit card too. Ask for a RIB :
« Bonjour, je voudrais un relevé d'identité bancaire s'il vous plaît ».
Your bank usually gives you several RIB because it's very useful in France.
You might be asked for a "RIB" or "RICE". A "RICE" is the same thing with the same uses, it only depends on the name of your bank.
Sometimes you can also directly print a RIB from your internet access if you have one or at a cash machine "imprimer un RIB" or "édition de RIB".