It's easy to get hung up on thinking about what makes a plot good or where to start to ensure your story impresses the readers. However, overthinking your plot will only muddle its brilliance and placing too much emphasis on creating a never seen before plot may trip you up instead. Some tropes are unavoidable in the face of making a plot point realistic and there are classic plot bases that are classic for a reason. It's not wrong to start off with an underlying theme people are familiar with as humans love feeling comfortable and are creatures of habit. Of course, there's always a balance to consider. You have a unique voice and the audience will want to hear you, not you echoing someone else. But how you do you get your special twist into your story?
An audience can tell when a writer loves what they wrote. The love shines through in the underlying tone. Have you ever watched an actor perform and knew they cared a lot about the role or the message of the film? We have a sixth sense for this kind of thing so it's important you enjoy your plot as much as your audience will enjoy it. That being said, a great approach to writing your plot can begin with a list. Try gathering things you like from books or T.V shows. Do you enjoy a quirky, outcast detective over a sassy, pompous one? Do you like chance encounters or do they bother you because they're too convenient? Try thinking about your favorite childhood book series or that movie you've re-watched five times. What are the patterns between your favorite media or the media you often re-visit? What's the patterns in the stories you hated or didn't understand? Once you gather your lists you're mind will likely already be rolling with ideas. Once you know, for example, that you want a loner high school student, a chance encounter with a supernatural being, a plot twist where someone reveals they've been hiding their true identity, and an epic climatic fight scene, then you already have the base for a plot and the details will start to fill themselves in based off the logic surrounding those points and the needs those items have (for example, a student needs a school setting and a secondary main character needs a different personality than the loner main character).
It also helps to free yourself. If you want to write a YA book but are worried about fitting in to the popular trends and therefore start bending your story to be more like other popular series, the quality may turn out to be not what you hoped. If you want to write a comedy script but are worried because some friends of your's don't get your humor and others don't like sex jokes and so you manipulate the humor and censor yourself hoping to please a wider audience, you'll have lost what may have made your story "you" and it won't be as enjoyable to write. Without heavy external force, it's hard to finish writing a story you wouldn't enjoy reading. While often writing is also a job and so creating a marketable plot can be important, placing too many constraints on yourself may still do more harm than good. Trying freely writing what you want to write without a care of what your parents will think or if it will be too niche for a publisher, and you'll come out with a gem that someone somewhere will surely value. Once you have an idea you're truly excited about, use your lists of likes and dislike as a guide and trust your brilliant mind and instincts to fill in the rest of the plot.
Plot building is often the most fun part of creating a piece: enjoy it to the fullest! Also, don't be scared to throw in some off the wall ideas or change things up even at the end. The great thing about outlines and having an editable work is that you can always go back to earlier parts and change or add things to fit your later ideas. Time flows differently with the creative process, so embrace its flexibility and trust your tastes!