It will be a script based rpg, probably no combat just puzzles and investigation. Also i have to say that i only played DnD but no idea of the version.

I already have the idea but what do i need to specify? I can write the background, the time and society but in order to "guide" the players should i have a plan for the decissions they can make? Like building a tree map with most of the possibilities?

How do you usually plan a rpg?