If I bit into a burrito at a fast food chain and it broke my back lower molar, how can I get the restaurant to pay for my bill?

First, you would have to determine what it was in the burrito that caused you to break your tooth and you would also have to show that the restaurant was negligent in having whatever this object was in their food.  If you still have the object, hang on to it.  Hopefully you made the fast food establishment aware of the incident when it happened.  I would suggest you contact an attorney in your area who specializes in personal injury and get a consultation.  He or she will be able to hear all of the facts of your case and give you a more informed answer as to what your legal rights and options may be.