Each week, I receive several requests, asking for advice on what to write.
It's an impossible question for anyone to answer, including the person who's asking the question.
If you're not sure what to write, just start writing. Sooner or later, you'll have your answer, because your writing will turn into: an article, the start of a book, a short story, a novel...
Writing is a lot of fun, when you allow yourself just to write. Writing is always a process of discovery, so you don't have to know where a particular piece of writing is going when you start to write.
If you want to write for money, the answer's a lot clearer. Write what people will pay you to write.
As I said in this article How to Write for Money Online - Choose What You’ll Write — Angela Booth's Fab Freelance Writing Blog:
"When You’re Starting out: Get Paid to Write for Others
Every business of any size - and many small businesses too - needs words. Finding those businesses is easy. Just go to any of the out-sourcing sites, and bid on projects. Write the projects you win, and ask your buyers for testimonials.
Those testimonials are vital: once you’ve gained some testimonials, you can make more money than you ever could on the out-sourcing sites by creating your own Web site, and selling your services there, directly to buyers who approach you."
Here's what to do when you don't know what to write
Anytime you feel the urge to write, just grab a pen and some paper, or create a new word processor file. Then just start writing until you want to stop.
That's truly all it takes.
Sometimes what you've written will amaze you. At other times, what you've written is nonsense.
However, the next time you sit down to write you'll find that the previous writing session has loosened you up. You're suddenly inspired. You couldn't have reached that blissful state of inspiration without the previous "nothing" session of writing.
So when you don't know what to write, just start writing. :-)