The easiest answer I could give to the question, "How do I find writing ideas?" would be to point you in the direction of my five books on the subject. But that doesn't really get to the heart of finding ideas. Creating stories and brainstorming the building blocks is a state of mind. There are some writers, myself included, who tend to complain about having too many ideas. That's because we've gotten into the mode of looking for ideas anytime and anywhere. It's a faucet we can't turn off. My hopes with this post are that I'll be able to provide you with a similar faucet.
One of the first places I started pulling ideas from was my life. In my junior year of college, I wrote a play about my junior year of college. It was exaggerated and the names were changed, but I pulled the plot lines and characters from the people around me. I wouldn't recommend doing this as directly as I did (some of my friends didn't appreciate their representations), but sometimes the most emotional and realistic writing contains multiple elements of a writer's life.
Start looking around for people, places and events from your life that would make interesting writing. You can write about the stories as they happened, or you can adapt them in any way you please. You can condense time, combine disparate events and even mix several people together if you want. You're the writer, and that gives you the freedom to choose the story that you're writing.
When it comes to finding writing ideas, you don't have to just match real-life events and people together. You can take the stories you read and watch and mash them together to your heart's content. Suzanne Collins, the author of the bestselling Hunger Games series, famously flipped back and forth between two channels portraying war coverage and reality television. She mixed the two together and Katniss Everdeen and the Hunger Games were born.
What intrigues you? What are your favorite stories of all time? Who are two people or characters you'd like to see interact? Mix things together until you come up with something that makes you excited to write about. You may not come up with the next bestseller, but that isn't the goal here. The objective is to come up with a book you'll start and finish.
Now, there are those who feel that every idea should be completely original. I used to feel the same way. In improv comedy, there's a mindset that if you've ever done a scene or a character before, it's off limits for performance and you should scrap it and start over. When you're trying to find writing ideas, this way of thinking can be counterproductive.
Nearly all of Shakespeare's 40+ plays were adaptations of other works. Sometimes his lines were word-for-word copy and paste jobs of other writers' work. Do we revere him any less for that? Of course not. He knew the trick of adaptations. You need to leave the work better than when you started. As long as you're adding something or mixing two works together in a way that results in a stronger finished product, nobody is going to care that you borrowed an idea from somebody else.
Critics are always going to compare what you've written to the works that have come before it. That just comes with the territory. Don't worry about what people will say if your story resembles another in multiple ways. Some of the best ideas came from the improvement of old ones. If we always had to start from scratch, there's no way our stories and our society would be as far along as they are today. Don't be afraid to borrow from the ideas of other writers, as long as you're making them your own.
If you think of a strange writing idea before you go to sleep, you should write it down. If you come up with a character that doesn't work for any of your current stories, write the idea for him or her down. If a book or a movie catches your eye and really gets the writer in you excited, make a note to watch it again with a pad of paper. You may not always know when the foundation of an amazing idea is forming. Your brain does. That's why it leaves you little clues along the way. If you don't write these clues down, if you don't follow this evidence, you'll miss out on what could be something incredible.
Always carry around something you can write on. It can be a notebook or a notebook app on your phone. Just make sure you get the ideas saved somewhere that you can and will view at a later date. The small ideas you come up with today may be a part of larger ones down the road. Trust yourself. Write them down and watch your writing ideas take shape.