So you want to be a writer? You have an excellent idea and it is going to be an amazing book. GREAT! You write that but then think, I should write another book....
How do you find something to write about?
Look everywhere. Ideas can come from multiple sources. Friends, news, science, politics, travel, or a twist on a classic---but a big twist, no copy, pasting.
You can grab part of one, say a gene they isolated to cure skin cancer (no not really, I am making this up) but it can have side effects---say, it can make you invisible. How could that work for the army? In the hands of the bad guy? What does the hero need to do to solve the problem? Maybe the hero is invisible himself---one of the first in human testing.
So how easy is it to come up with ideas? It takes practice. You shouldn't hold back yourself if you think you are just being silly or it's too crazy you can always strip away the extra and you might surprise yourself with something really great. Just go with it, brainstorm and write down everything that comes to mind.
Remember it's not only about coming up with great ideas, it's about turning those ideas into stories. :)