Writers are often asked, "Where do you get your ideas?" The answer is easy: ideas are all around us.
You discover them in what your read, what you hear (writers are notorious eavesdroppers), in what you see, and so many other places. But an idea is not a story. An idea is the germ of a story. It's what gets you asking, "what if..."
The next step in the process is creating character(s), a plot and a story location. There has been argument over which is more important--character or plot. In my opinion, they're equally important. You can't have one without the other.
For instance, you want your main characters to have substance, not be paper cutouts. Readers can relate to a realistically portrayed character. So how do you do that? You give them lives. You describe them, their characteristics, their personalities. Many readers are turned off by two much description. Some want no description of a character so they can employ their imagination as to the character's appearance. My feeling is it's your story. Their imagination may not match your vision. That's why I prefer to describe my key characters.
Others will quote Elmore Leonard's famous 10 Rules for Good Writing, citing Rules 8 and 9, which tell you to avoid detailed descriptions of characters and not to go into great details describing places and things. The important points are "detailed" and "great details." He doesn't say don't describe. Read Leonard to see how he subtly introduces characters and place, so aptly we feel we'd recognize the character anywhere and know the place even if we've never been there.
So you don't want to give them everything about your character in one lump. Introduce details gradually throughout your narrative.
Now, as to plot, this is the narrative of your story. It introduces the crime, the detective, the investigation, discovery of the motive(s) and, eventually, the identity of the culprit. Some mysteries disclose the identity of the killer at the beginning, but I think that takes the fun out of the story for many readers. Most like to try and outsmart the writer, determining the identity of the criminal before it's disclosed by the writer. Traditionally, mysteries started as this type of puzzle, providing clues through the narrative to lead the reader to the conclusion. Being the sneaky people we are, we throw in red herrings (misleading clues) to throw the reader off the track as well as sub-plots to add a little more substance to the story.
This is how an idea becomes a story.