I've begun editing this (10.09.2017), but I feel I should point out that this is a bildungs roman-ish endeavour, which means his writing and vocabulary gets better the older he gets, because it's unrealistic for him to write like an adult from the outset.
Enjoy my first ever multi-chapter fic! I hope it makes you giggle :)

Dear Diary,

Good evening. My name is Namikaze Minato and I'm six years old. My mommy gave you to me as a 'Congratulations on getting into the Ninja Academy' present. She said that ninjas find it very difficult to talk to people because they have to bottle up their emotions and then they go insane and go on 'killing sprees', so I should write down what I feel in this to avoid that.

And since mommy's always right, I'll do exactly that.

As far as feelings go, I'd have to say I'm a bit nervous. I'll be older than most kids my age because I entered late because my family wanted me to do what I wanted and wanted me to want to be a fisherman like daddy.

I like fishes, but I like my village more.

So in the end, they agreed to let me enter the Academy and now I have to hurry up and get promoted otherwise I'll end up with a team a whole year younger than me, and then they'll make fun of me and call me a girly flake just like that really pretty girl on the street last Tuesday.

I don't know how it's going to turn out, but I hope it goes well.

The dictionary says that that means I'm optimistic.

Thank you for being really patient with me, Diary. I'm sorry if I was rude somewhere in between. And in case someone reads you, I love my mommy and daddy and fishes and any person and any thing I wrote about.

Good night Diary,

Namikaze Minato