If one was to attempt to breathe fire, the recommended fuel would be kerosene (or paraffin if your fire breathing internationally) because it has a relatively low flash point (the temperature at which the fuel ignites). Personally, I would not attempt to breathe fire myself - the thought of swallowing flammable liquid and spitting it into a flame a few feet from my face doesn't seem enjoyable.
However, if you are interested in possibly burning your face off and/or dying, here's how they do it. I am in NO way suggesting you try.
For a "safer" way to breathe fire, cornstarch can be used, as seen here.
Regardless of how you do it, I believe fire breathing is something best left to the professionals.