Don’t start big, just think of a simple adventure that can be run as one-shot with pre-gen characters. A simple fetch quest would do.
This is teaching them the game so don’t overthink it.
If they enjoy it and want more then you can think about making characters and a longer story.
I absolutely second the idea of using pregens. Here is what I do when introducing new players. About a week before, I e-mail the players a list of 10-12 two line writing prompts for characters that still leave room for customization:
The Paragon - at a young age, you heard a voice exhorting you to lead and help others. You have trained with sword and shied to achieve this command and supplement this with a little magic.
“The Exile” - exiled from dwarven lands due to your refusal to conform with the stifling dwarven traditions, you make your way from town-to-town calling upon your arcane power to keep you safe.
That way, you only have to create characters for those who are chosen.
In terms of adventure prompts, if they are not really into fantasy I would take that as an excuse to lean into some less-traditional adventures.
The following writing prompt is inspired by Shemshine’s bedtime Rhyme in Candlekeep mysteries.
Spoilers
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The characters arrive at the inn after a long day of riding. As they check in, they notice the innkeeper is humming a very catchy song. Pretty soon, everyone is humming it, and even if they stop, they immediately start up again if they stop concentrating. One of the patrons blocks the door, concerned that whatever this is, it is better to contain it inside the inn. The characters must solve the mystery.
What happened: the rhyme is contained in a book and will summon an evil fey if it is continued long enough. As the rhyme continues, it causes increasingly challenging effects to manifest in the inn, culminating in a battle with the evil fey itself.
The book also contains clues on how to permanently defeat the fey. It was brought into the inn yesterday by an unfortunate traveller, who realizing the risk, killed themselves rather than potentially bring the fey into the world. Unfortunately, the innkeeper’s daughter heard the rhyme before the traveller killed himself and continued it.
The innkeeper, worried about the reputation to their inn (and not understanding the situation) hid the body in the cellar and locked the door. They have no reason to associate the rhyme with the traveller.