What to Write
Probably the best answer I've heard or read regarding this question was given by Arthur A. Levine (the U.S. editor of J.K. Rowling's Harry Potter series) during a talk with the Florida SCBWI, the text of which is on his blog. Essentially he talks about plastic flowers and channelled raisins, which is probably causing most of you to go 'Huh?' right now, so allow me to elaborate.As you might've guessed, the plastic flowers and channelled raisins is a metaphor alluding to two very specific mistakes that most author's make (including your's truly!). The plastic flowers bit refers to the fact that many new authors submit manuscripts for publication that are just that, plastic flowers. They are not real flowers and, as everyone knows, its very easy to tell the difference between real and fake flowers, especially when you are as close and intimate with flowers as you are when reading a book. A 'plastic flower' novel is one that is not the real, true story of the author. Every author, budding or established, has enormous potential and inside them is an authentic 'real flower' story that no one else can copy or duplicate.
According to Arthur, these plastic flowers represent a syndrome that he refers to as channeling. This is where an aspiring author sets off to write his/her masterpiece and instead of writing their own masterpiece, end up trying to make their story as good as, or along the lines of, someone elses. (Pooh! - I've been guilty of this before.) In the process, they also pass along the same raisins, the same priceless nuggets of sweet revelation, found in other literary works (hence channelled raisins).
I probably don't need to go any further, but just in case some of you aren't quite following me, I will.
You should write about what you want to write about. Interestingly enough, this causes most budding authors to do quite a bit of thinking, but will be well worth the effort. What are your passions, thoughts, and desires? What is the ultimate human struggle? Perhaps for you it's not about struggle at all, but about the peaceful blissness of everyday life and long-lasting friendships. Whatever it is, it should be uniquely you. Even if the topic has been written or touched on before, no other person can give it the special perspective that you have.
And in the process of finding and writing about you, your story will naturally be full of your own authentic raisins; those sweet morsels that readers love to uncover, that make them covet and protect the story you have told as their own because it speaks to them so closely.
This, according to Arthur A. Levine (and I quite agree with him), is what to write about. And if you're still not sure what it is you like, think about all the books you've read. What are the novels that made the most impact on you? Which stories still have a place on your bookshelf?
Hopefully, this will get you pointed in the write direction. And remember, even if you don't know what it is you want to write about, just get out some pen and paper and start jotting down anything that comes to mind. You'll be amazed at what your creative mind comes up with.