well I mean you know fires dont come out of nowhere. they need fuel to keep burning so where the absolute HECK does the fuel come from??? does charmander pick up fkcing twigs off the ground and use them on its tail??? does it somehow produce oil in its body and it goes through a tube to its tail fire??? I cant even
I found your answer in the Chimchar pokedex entry in this own website --> "Pearl:Its fiery rear end is fueled by gas made in its belly. Even rain can’t extinguish the fire." Same goes for charmander.
From this you can see that some fire types create flames in their body using that method.
How about this: Charmander has a special gland near its tail that stores flammable gases, such as methane or hydrogen (which many animals naturally produce anyway), next to an organ capable of producing a small electrical charge (similar to eels) that's just strong enough to ignite the gases as they are released from its tail. Charmander also has similar glands at the back of its throat in order to produce flamethrowers.
By this logic, it would be fairly reasonable to suggest that the electric charge would not be strong enough to manifest itself into an attack, which is why Charmander isn't also an Electric type.
That's like asking how humans keep their bloodstream working or their heart beating, it's just part of how its body functions. It's a fire type Pokémon, obviously fire is integral to its genetic design. There are scenes in the anime that show the fire on its tail grow smaller as it grows weary, or larger when energised.