Scenes, characters, situations that people find themselves in. You should be seeing these things on a regular basis, and writing them down. And if that scene, character, and situation is something that holds your interest. That is when you should consider expanding it, and developing it in to a story.
You may well ask, where do I find such ideas?
If you are a writer, you shouldn't really be asking this question.
Read books, comics, novels, poetry, magazines, news articles in papers, the internet.
When you overhear a conversation, or while in a conversation. Something may well spark that light-bulb.
Watch movies, TV shows. Ideas are all around you. That's why should carry a little notepad around with you in order to write them down, and if they stick with you, you'll probably end up using them in a story.
You see something that interests you while out walking. A burned out car, a battered bike. A homeless guy carrying some bedding. There are endless ideas for stories everywhere. Maybe you look up at the sky one night and see a meteor shower, or flickering lights. What are they? Where are they going? Is there an idea (concept) here for a workable story?
The hard part is figuring out which of these ideas and concepts is workable.
Some ideas just aren't workable, while many are.
If you really are a writer, this is not the question you should be asking.
There are no shortcuts.
Best of luck with finding those allusive ideas.