Helpful Links:
FiveM Wiki: https://wiki.fivem.net/
Tutorials: https://forum.cfx.re/c/development/tutorials
How to create a basic script by Scammer: [How-To] Create a basic script
Native Functions: http://www.dev-c.com/nativedb/
I would also recommend you take a look at some code examples by downloading released source code from other members. You can find released scripts here: https://forum.cfx.re/c/development/releases
Misread question, sorry about that.
As for the Scorpion Trainer, you would edit the trainer.html and trainer.css files. These are what show and style the menu, this requires HTML and CSS knowledge. To change the functionality of the menu you need to edit trainer.js which is what controls the navigation/displaying of the trainer. This is using JavaScript.
To add new functionality, say make yourself invisible, you need to take a look at how it is handled in the player.lua file. The action value is pulled from the trainer.html file and the data-action attribute of its elements.
EDIT 2: I’m assuming your name doesn’t have anything to do with your question, if it does then make sure you named the folder trainer and added - trainer to your citmp-server.yml. This is case sensitive.