1. no, no training on fire.
2.fire is weak against water because water cools the flames, and strong against wind because wind fuels the flames. i would combine fire with wind because the wind would make the fire attack stronger, and i would also use earth to shield the fire attack.
3.in real life, fires is caused by a bold of lightning, something exploding, or when the sun hits a dried field/forest.
4.i want fire because im going to make a bio that mainly uses fire tequniques.
5.
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amaterasu because it was the very first fire jutsu i ever saw