story – Newbie To Tabletop RPGs who want to make my own

I already have a lot of things for my RPG, but I'm not entirely sure about the story, and I have no other way to prove it than with friends, which are not always my will and my whim.

The setting is a rpg of Sci Fi Space. My principle thought for the story is that Terran and Hequns (an alien race) are in diplomatic conflict over the territory.

I want it to be a kind of story, but it's not always easy to think about all this.

I wonder if I should develop the story while I'm playing the role that I know, or if I should develop a complete outline for it.

I think the mechanics I have can be pretty good, but I still have to try them.

Anyway, I would like some advice for my questions, and possibly a tip for being a GM noob.