These are the BASICS of the shinobi world. Students in the academy are graded on this, and will continue to be graded on it even as  jounin.

Some characters will be good at all (prodigies or hard workers). Some will be good at one or two (they will devote specific training to that field). Some will suck at them all.

Your character NEEDS to know their strengths and weaknesses so they can hone it and try to minimize the weakness or whatever.

Naruto characters will choose which, or how many they will specialise in. Will they be good at ninjutsu, taijutsu, genjutsu or intelligence. Will they be good at two? Three? All? None?

In this section I will talk about Ninjutsu : it's types, how it relates to ones elemental, the skills one can obtain in ninjutsu and RANKS of ninjutsu.

Taijutsu : forms of fighting, the basics of martial arts, and the four cores of taijutsu (speed, strength, stamina, dexterity and defense), RANKS of taijutsu

Genjutsu : how it works, RANKS of genjutsu, how it affects the mind

Intelligence : Basic brain tests to use on children and adults and how the mind works in battle

And, lastly

How to know which your character would be most suitable for in terms of plot, genes and personality match.