Most of the time when you don't what to write about is when you're afraid it won't be perfect.
Develop the discipline to write even when you don't feel like it. If your aim is to make it your full time profession and to make money, there will be good days, and there will be shitty days, and on all of those days you'll have to write.
So show up, get your butt in chair, and do it.
Journal first, if you have to warm up. Or (my favorite) write about something that bothers you at the moment. For example, it might bother you that you don't know what to write about. Then write about how awful it is to not know what to write about.
In a few minutes you'll forget all your troubles and get sucked into writing. Hell, you might even come up with a short story about a procrastinating writer who never knew what to write about and ended up in an asylum.
P.S. I wanted to continue telling you how the writer shredded the paper and then made multiple paper-cuts on the skin in frustration, but then that would be me and my kinds of stories. Funny enough, I started writing this post while thinking, "Damn, I don't know what to write about," and then a whole story formed in mind. Go figure. I guess that's how we writers work.