Human beings are sponges and artists and writers even bigger sponges. We are constantly absorbing all the input that is rushing into our febrile brains; or we should be if we want to call ourselves artists. Observation is one of the greatest tools of art. We need to be looking around at the world around us all the time and taking it in, adapting it for our own creative impulses. I am influenced by the news, movies, books, other people, nature, you name it… and all that stuff feeds into my brain, gets melted down, re-mixed, and spews back out in the form of words, ideas, and colors (when I'm painting). I paint when the ideas are non-verbal. I write when the ideas are stories. It's ok to be influenced by what you have read or watched, but it's always good to take a long hard look at what you produce and ask yourself is it 'influenced by' or is it completely derivative? With all this constant feedback coming in it's sometimes hard to know when you're being original. Bearing in mind that you don't want to be a copyist remember that all literature and all art (movie, manga, pottery, whatever) is an ongoing conversation between creators and viewers/users. There is no single pristine piece of work that has never been touched by a previous idea. Even the Lascaux cave paintings are working off previous ideas.
They say there are only seven plots, and if you flense it to the bone that's probably about right so you can never be entirely original. Don't sweat it. You still want to be careful though that you're not just picking up something in its entirety and putting your name on it. Even fan fiction can be entirely original and yet be playing off of someone else's riff. Find your voice, your own ideas, and pull it in.
So back to ideas: I often get an idea from another work of art or entertainment. Often a movie will give me an idea for a character. I like the movie, I like the character, but what I really want to do is to pull that character out and put him somewhere else and see what he will do. And while the original movie I viewed might be my jumping off point by the time I have pulled in other source material it's all looking a lot different. It could be a book or a story I read in the newspaper, or even a memory of something I loved or was intrigued by twenty years ago. Some ideas have a much longer gestation period than others. I might buy a tube of paint, some particular hue I haven't tried before, and it might sit unopened in my paint tray for a couple years before it becomes just the right color for the job at hand. There is no rush to use up ideas.
Don't treasure ideas too much either. As I have pointed out they are unlikely to be entirely new and original to you. Ideas are like colds. They like to be spread about to bring joy to each new carrier. Someone might easily have the same idea or a similar one to you and they didn't have to even poach it from you in the first place. One idea is too trite and overused? Find another one. They are all around you. In fact they're literally littering up the place if you just open up your senses and take in all the data.