Anybody who has written for very long will one day hear somebody say, "I would love to be a writer, but I can never come up with a good plot." If you are one of those people who has trouble with plot ideas, here are some suggestions.
Characters and Setting
I believe that what we call plot is one side of a coin. I read somewhere that the plot is what your characters do while completing their story. The story is their emotional journey, the part of the tale that matters most. Often, once you have your main character and her goal, and a person who stands between the character and her goal (often by wanting the same thing), the story ideas start flowing, and the plot naturally follows the story.
Jenny hid her licorice from her big sister Mona, and Mona wants to find it. In this scenario, you could choose either of these girls to be your protagonist, and the other to be your antagonist. Once that is sorted out, plot ideas suggest themselves. Drop them into an unusual setting, and more ideas pop up.
As long as two characters want the same thing and are willing to do whatever they can to get it, or one character wants something and another is going to do anything possible to stop her, you have a story that leads to a plot. Add interest by putting them in a setting that fires the imagination. The ideas will seem to spawn themselves.
This is why authors, no matter how carefully they outline the course of their tale, are so often surprised by their characters' actions and end up going in unexpected directions. Ideas have a way of creating themselves as the characters move through their emotional stories.
Countless books and short stories have been inspired by the news, either current events or things that happened in the past. Using a story from the news doesn't mean you have to write about that event. A botched bank robbery in your home town can easily become a failed attempt to steal space ship fuel rods on the planet Gor'k'h'dt in the Dutnam System. If a story in the news or in a history book fires your imagination, play with it. Twist it. Turn it on its head. You might be thrilled with the result.
Other Books and Movies
There's nothing wrong with finding your plot elements in another story. Storytellers have done that that since the beginnings of storytelling. You don't want to plagiarize or steal another writer's stuff, but you can feed your plots with other stories. Look at how many stories have been inspired by Shakespeare, many of which would surprise you. Many stories have also been inspired by fairy tales. And any time there's a hugely popular story like The Lord of the Rings or Harry Potter, similar stories pop up. How many vampire romances have you seen in the last few years? You can get countless stories out of an interesting concept, even if it was originally somebody else's.
What It Really Comes Down To
When somebody says they can't come up with any ideas or plots, what it really means is they lack confidence in what they do imagine. That inner editor doesn't start popping up after you start writing. Too often, as soon as that first thought of a story sparkles enough to escape into your conscious mind, doubts start. "It's been done before," or "my grandmother wouldn't approve of that story," or "I'm not good enough to carry it through"--these are just some of the countless thoughts that can spoil a perfectly good plot idea. Trust your mind, and trust your imagination to take the plot into a direction that interests you enough to keep writing it. If it interests you, there are readers who will like it too.
Plots are all around us. Good ones. It's been said that there are really only a few plots in the world. The number depends on who is saying it, but let's use the commonly cited number four. (Lately it's seemed like there are even fewer than that, especially in Hollywood, but let's not go there today.) The things you can do with those four plots are endless. How can that be? Because people are endlessly complicated. It really comes down to that emotional story I mentioned early in this post. Find rough outlines of two people's whose goals conflict, then pull from all the stories around you to give them things to do, and the possible combinations are endless.
Remember, we also have only 26 letters in our alphabet, but those letters can be rearranged to become all of Shakespeare, Homer, all of the world's scriptures, the Gettysburg address and the Constitution, The Adventures of Captain Underpants, and a truly infinite number of things. Don't even worry about originality. Nobody else thinks just like you. Whatever you write will not be identical to anything else that has ever been written or ever will be written again. So take the plot elemnts that pop into your head, whatever the source, and have fun.