Ooh, serious. XP That's not a good nature at all. You should read the stickies at the top of the strategies and movesets forums, but here's a quick summary of what you should know:
Natures: Every pokemon has a nature. Natures raise one stat by 10% and lower another by 10%. For example, the Adamant nature rasies Attack but lowers Special Attack. A modest nature does the opposite. Serious nature, however, gives you no stat boost, and that is highly undesirable in your training.
If, however, you still want to use your Charmander, you could train it. Shiny pokemon are usually just as strong as normal pokemon, so unless you're striving for a really good pokemon, go right ahead and use it.
Now, EVs: EVs, or Effort Values(sometimes called EPs, or effort points) are given out by pokemon when you kill them. For example, killing a Starly gives you one EV point in Speed. Get four EV points, and your stat goes up by one. You can only get 252, or a +63 stat boost in a particular stat(at most) and you can only max out two stats in this way. There are items to speed this up, and a disease called Pokerus, but you can learn all about that and much more by reading the stickies.
Oh, and BTW, hatching an egg in Mt. Coronet doesn't increase your shiny possibilities. There is always a 1 in about 8,000 chance.
EDIT: These sites will be very helpful if you choose to continue:
https://www.smogon.com/dp/pokemon/charizard
https://serebii.net/
