Want to write a story about two young lovers who fall for each other even though their families are at odds? Shakespeare already did it. How about a grittier tale, where one jealous brother coldly murders the other? Sorry -- that story's so old, it's in the Bible. There's a theory of belief out there that there are no more original ideas...and I happen to agree. If you're waiting for an original idea before you start writing that book, stop.
It's All Been Done
When it comes to basic plot, every single sort of love story has already been told. Every drama and comedy was written long ago...most of them by the ancient Greeks. And mysteries? They're already solved. After all, there are only so many ways to kill.
Read the Bible, the works of the ancient Greek playwrights, and Shakespeare...and you'll see that there's really nothing new in the world of fiction. It isn't your plot that needs to be original, but your take on it does. When you write, you should be doing so with your own unique voice and style. That's something that can never be duplicated by anyone else. Even Shakespeare didn't always write original ideas (you'll see it after you read the Greeks), but his work is revered because it's good. It's distinct because he made it his own.
When you do the same, it doesn't matter how unoriginal your best ideas might be.