It will be a script-based RPG. Probably no combat, just puzzles and investigation. Also, I have to say that I only played D&D (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 the society, but in order to "guide" the players, should I have a plan for the decisions they can make? Like building a tree map with most of the possibilities?

How do you usually plan a RPG?