/lit/ - Literature

Newfag reporting in. I'm posting to see if it's allowed to get help for our writting here.
Context: I'm writting a mission based 3D game (much like GTA if you'd like). And I'm having a hard time writting the story. Everything was going well for the first arc (the story is cut into 4 arcs) but when I try to get ideas for the others ones, it progressively degraded. I had some stuff for the second arc, a general idea for the thrid one. And the fourth is only "Beat the final boss" for the moment.

First, do you have any advices on how to write shit, it's like the very first time in my life I'm doing this. Especially on how to fill the gaps, make connections and iterate on the story to add more layers of scenario and relationships on each iterations.
Then, I'd like to have some recommandations based on story I have. Wich is the following (condensed):

MC is a farmer boy who freshly moved into a new city to attend high school. Being a farmer boy, he could only afford to attend one of the worst school in the country, filled with delinquents. MC is a kind soul but he's quickly drawn into this Yankii world by being mogged of a new goodies he just bought from his favourite show. To add shame to the pain, he tried defending himself but quickly got beat up and ended having a photograph of him taken in a funny pose (pants down or whatever, something stupid like that).
First arc is about getting that goodie back. So we realise that there's an entire mugging team lead by the most prominent gang of the school. We get introduced by the yankii world and who's leading/dominating it.
The second arc start by the leader of the gang learning that his "mugging" captain got beaten by a random farm boy. The MC isn't taken as a serious menace for the moment so he just decide to take our reputation down by displaying everywhere in school (and/or in town), that photograph from the begining.
So the main goal of the arc is to find the original owner of the photograph and stop the production of the photos/posters (there's a substory about the guy that produces the photo and all).

This is where it gets poor in content. Third arc, the leader is pissed that we took his second captain out. Two being the ones the brang the most money. He has a new card though and a good one. One of his old captain just got out of juvie. He spent quite a time there for a serious crime (don't know wich one yet). So he just send him to take care of the MC.

Fourth arc: Confrontation with the boss and beat him.