Hello! Welcome to the site!
As this is your first storygame, I'm going to repeat the most useful writing advice I've ever received: when you're starting out writing, don't write about your character's entire lifetime or career--just write about a single day in their life and make a game out of that. You'll learn the ropes, and you can build from there. This advice was the tipping point in me finally finishing my first project. (Here's the article I got that advice from.)
As far as character customization: It's certainly doable, but in allowing each player to customize their character, you sacrifice your ability to tell certain specific and engaging stories. On CYS, the general attitude is that people prefer stories with distinctive and well-developed protagonists than customizable ones.
Here's the article home page. This should teach you everything you'll need about variables and scripting, if you're confused you can ask for clarification in the advanced editor forum (but most things you can figure out just by troubleshooting a little).