Sort of unclear what you are trying to get, however some light research can go a long way.
Fire needs fuel, oxygen, and heat in order to survive. Oxygen is prevalent in the air outside of the body, as well as would be inhaled into the body. However, if the fire is created inside the body, then perhaps the lungs would need to be larger. The next thing we need is heat. Now, the inside of a warm-blooded body already packs a lot of heat, but to actually spark this flame, the dragon would need bio-electricity, perhaps or the fuel used to make the fire could be pyrophorus to just skip this step. Finally, the actual fuel used to make the fire. There are many pyrophorus materials that can light up on impact with the air, but I found that potassium also reacts violently with water to answer your second question.
In order to make the dragon resistant to it's own fire, you could have a layer of argon covering it. under the argon, however could just be fire-resistant hide or scales.
The only problem I see with this is how the dragon would actually obtain all of these metals. You said that it was sort of modern, so maybe the dragon could steal the metals from research labs and factories that contain it?