True. In fact, there are only a handful of basic plots. But none of them have been done the way you will do it, with the characters you will create, and the situations you will put them in. I was at a writing group last night, where we had to develop a character for Alex, a 50 year old introspective bus driver with a habit of finger clicking. There were twenty people there, and they all wrote entirely different things. Alex varied from being a nun and an alien to a drunk and homeless lady. Just think how much differences there would in how we treat a plot idea.
EVERY single good idea has been done before. It's your job to make your reader THINK your story is new. You do that with characters and setting. Maybe to get some ideas going you can take some well-known stories, and change one decision a character made. In The Lion King, what would have happened if Simba hadn't run away? And don't just say "Scar would have killed him," be creative!
Just tell a good story, and don't worry about how many other stories follow a similar line. Make your characters interesting, and learn to play with the reader's emotions. Learn the difference between plot and storyline. Set up the situations and the characters, and start writing. Let the ideas come from exploring how your characters respond to crisis.
I'll use metaphors to illustrate the points I agree with. Like EileenG pointed out, there are only a handful of basic plots. Think of these basic plots as car engines (V4, V6, V8, etc.). Now, there are hundreds of different cars that have the same engines, despite being completely different. In other words, core-wise all plots have been made, but it is up to you to make it unique through the "car parts."
Maybe you could just sit down and write whatever comes out. Let it go wherever it wants and then just fill in the blanks. I don't know just an idea...
Masterplots- "plot" refers to a single narrative's story- are extremely useful. They let the audience know what's coming and what the story is about, they let the author trick the audience into thinking they know what's coming and what the story is about, and they supply the author with a handy framework for knowing where the story should go next. Don't think of them as negative structures or premade plots. Think of them as a building plan with most of the rooms already drawn, but none of the furniture filled in and none of the materials bought. In other words, they are too simple, too big, and too useful to claim that they are overused. It's like saying that words are overused, or that black ink is overused. Yes, you can dodge around them, but let's face it, you'll still go back to black ink and words in the end.
If you feel strongly on a subject, just write. Write whatever you're thinking when you start thinking it, and afterwards maybe you can put it all together. That's what I do.
Theres a good episode of South Park that all writers should learn from. The episode is mostly about how Butters tries to do something the Simpsons haven't. Oh and Cartman ruling over an advanced civilization of sea monkeys or something. Butters goes crazy and at the end learns a valuable lesson: Everything's been done before. Just because its been done before, doesn't mean you shouldn't write what you want. Make the characters, setting, plot, storyline, your own. Just because the plot has been done before, doesn't mean there isn't anything left to explore. It doesn't mean you can't write a great novel. But thats just how I see it.
That actually is good advice, Unit. Either you make things that have been done already your own, or try to use personal experience and make things more unique....either way, if it's well-written, you can't go wrong.
There's nothing much left to say since everyone here has pretty much said it. So, I'll just repeat it because it can't be said enough time. True, every plot has been taken but that doesn't mean you can't make an original story because you can; there will always be areas left to explore. Make it unique, personal to you. As long as the characters, their actions and motives etc. are well-written you'll be sure to find readers. Take the vampire theme for example. As a theme I enjoy I have read hundreds of stories, some as books I have brought, others on the internet. Often, I will groan and say 'not another human-vampire romance!' but even they have little details that I perhaps don't come across often and every now and then I will find a very unique version. Whatever happens, you mustn't ever say 'this is useless, I'll never find originality' and give up. Writing provides you with endless opportunities and it's up to you, as the writer, to grasp them and use them.