Need help! Trying to create first plugin!

Hi everyone,
I need some help.

I’d like to learn how to write my own plugins but I’m just starting out and don’t know how to add user profile fields (I think they are called ‘meta data’) and how to refer to those when I want to.

I’d like to create a referral plugin that does two things:
1) When someone refers a guest and the guest accepts the invite, both get a coupon.
2) From then on, when the guest buys something, a certain percentage will go to the referrer.

I am just a beginner: just started learning how to program last year. Discovered WordPress a few months ago and love it and the community

Any advice would be greatly appreciated.
Thanks guys.
-Gabe

https://wordpress.org/plugins/woocommerce/