When I'm watching a movie or reading a book, I'll sometimes play a game called, "Where did this start?" A lot of times, it's really obvious where the idea for something came from; the most obvious is the high-level description. Especially movies these days, it's all, "Newspaper boys that sing and dance," or "Like Rocky, but with robots," or, "Like the video game, but it's a movie." That sort of thing. Usually I don't play the game with the really obvious sources.
Sometimes, though, the source of inspiration isn't just the elevator pitch. Sometimes you start with the opening scene, and see what falls out of that. Sometimes you create a world, and you put someone into it, and see where it goes. Robert Jordan always said that the inspiration for the Wheel of Time saga was the ending scene (which is why it took so long to actually complete).
Sometimes you start with a theme, as often happens with school projects. Sometimes a snippet of conversation you've overheard, and you want to create something which manages to recreate that snippet. Sometimes an image, sometimes an emotion, sometimes a name.
Sometimes you want to change the world, and show everyone that what they thought was true really isn't. That's kind of the place that Buffy, the Vampire Slayer came from: turn the trope of the helpless damsel on its ear. Lazy writers use clichés as a way to get through their writing more quickly and easily; clever lazy writers turn those clichés around.
You can start on your story anywhere. Some people claim the beginning is the best place to start, but even if that were true, it's not the only place. Find a middle, or an end. Or pick a scene that will never even show up in your story, and work everything around that. Your choice, if you but choose.