You can start by observing the world around you. People you meet, places you visit, and things you do can all be sources of inspiration. For instance, if you notice an old, abandoned building, you could create a story about the secrets it holds or the people who used to live there.
Another way is to combine different elements from different genres. Take a mystery plot and add a touch of fantasy. Maybe a detective has to solve a crime in a world where magic exists. This can create unique and interesting story ideas. Reading a lot also helps. You can get inspired by the ideas of other authors and then put your own spin on them.
Dreaming can be a great source for story ideas. Sometimes the strange and illogical things that happen in our dreams can be turned into fascinating stories. You could keep a dream journal and write down the key elements of your dreams as soon as you wake up. Then later, you can expand on these elements to form a complete story.