HOWTO create a plugin?

In setting up my WordPress, I’ve made some modifications to files not in my theme directory. Most notably:

upload.php
vars.php
quicktags.js
kses.php

So I am afraid that when a new version of WP comes out, I’ll lose my changes to those files. ??

Should I write a plugin to make the changes that I want, since plugins don’t get changed during upgrades?

Am I understanding this properly?

Also, I have no idea what is involved in making a plugin, so I may be in WAY over my head. My abilities are enough to allow me to make changes to the above files, but I’ve never authored a plugin before.

What do people think about this?

THANKS!!! ??