Creating a BASIC custom model, python, trained with a prompt

Hi there I’m admittedly having trouble finding something that can guide me.
I’ve seen some more complex convoluted stuff, but I’m trying to keep things very simple here so I can learn.

I want to build a python script for davinci-003 model.
I want to pre-train that script with a prompt such as “I’m an AI who repeats everything it’s asked”
I then want to save that model so that I can reference it again.

So the expected results is that I could send “Hello World” to the new model that I created, and it should spit back “Hello World”

This is literally just a baby step to learning this stuff. I’m still pretty new and haven’t coded in a while.
I just want to create a custom model man, haha.

Any advice or references would be much appreciated, thank you.