shit, I'm starting to see how I suffer from zero actual creativity when trying to plan the story, so I'll try to rip off some game or something.
Don't overthink it, my first fallout pnp adventure (not my first, but ran for first - timers) consisted of three encounters - the characters were tribals who had to kill a radscorpion as a rite of passage,
after planning how to ambush it and doing the kill, they had an encounter with a slightly retarded super mutant (another player who came late) who threatened them with an empty minigun and wanted to steal their stuff
- this was a no dice and all talk encounter that the players basically made up on the spot. All in all, went pretty lulzy. Finally they were to present their exploits to the elders and at this point I had several hooks ready to lead them to whatever the main campaign might be. Sounds simple, but it was just right for one time.
Lots of memorable stuff will be totally ad hoc and the best you can do is stay on your toes, provide some general framework and let the situation develop on its own.
It's the players who fill out the gaps, so don't try to "plan" amazing stuff to blow them away. In the beginning it's the simple things that work. With experience, you can start building intricate metaplots and epic campaigns.
Another thing you'll see in good time is that people will run out of steam after 3 hours or so, because there's a limit for how long you can stay focused and act hings out (especially if its the first time).
Some time will be lost explaining the system, fiddling with the character sheets (levelups and all) and making idle chit chat. That's how it rolls, don't fight it.
If the session ends a bit earlier it leaves you some time to talk about what worked and what didn't.
You may discover that the setting or system doesn't work for you at all, or maybe someone would like to start over with a new character. Such technicalities will take some time during the first sessions.