Start with a germ, build to a story.
I don’t know about you, but when I started writing, I don’t think I really understood the difference between a concept and a story. In practice what this meant is that I would get super excited about a concept (vampire office worker, man-eating mermaids, etc.). Drunk with enthusiasm, I would dive in and write and write and write, and somehow I would never quite move beyond the concept to the story. Sure, I might have written a few decent scenes that captured the quiet desperation of my office drone vampire, but that was it. Nothing happened. Concepts by nature are flat, one-note flashes of inspiration until they’re fleshed out, but it took me dozens of drafts (a few finished, most abandoned midway) to realize why my stacks of manuscript pages never felt like real novels.
I was skimming the surface, taking the easy way out, suffering under the illusion that an idea is something that matters. An idea is something that’s been done before (at least when it comes to writing novels), that’s just how it is. Yes, even that glorious, bolt of white lightning known as your latest and greatest idea has already been written, probably a bunch of times. Once I realized that, I was able to move past the highs and lows of writerly inspiration and get down to the hard work of being an actual writer, i.e. starting with a concept and moving beyond it to uncover the intricate, deep, chaotic and truly original story hiding underneath.