Well... it depends on what kind of thing you are looking to write. For most short stories it's important to have a strong central concept, or a twist in the story. So if you can start with that, do that. "The guy trying to kill him is actually his future self who has gone back in time". That kind of thing, you know? You tell a scene, and then there is the twist.
If your trouble is that every idea you have seems to bring with it a long-form approach, hundreds of pages and volumes, the simple answer is, none of your ideas probably actually need that. You can tell a short about any of them simply by taking one core scene that would be in any of those books and telling it in an accessible way.
This is another approach for which you don't even necessarily need a concept with a twist or anything: the slice of life story. This has, after all, been a type of short story for a very long time. Imagine you have a longer story, and tell one slice of that. Could be a conversation between the hero and his nemesis. Or a crew discovering a habitable planet in a distant solar system. Take one moment, tell it in five pages. Try to figure out how to make it flow, and how to make it interesting.
More generally, you might try out different techniques of developing ideas. For sci-fi? Do what Warren Ellis does, read the tech websites and think about how this technology could develop in the future. Then, think of a short story for that.
Or imagine situations, or characters, that catch your interest. Or think of ten first sentences, and see if any of those gives you something to work with tomorror. Or think of your favorite novels and about how you could give those stories a twist.
There's a lot of creative writing techniques of how to come up with ideas; it's actually not that hard. Writing the actual thing is, on the other hand... but that goes for all lengths, whether it's 5 pages or 500.