The eternal question for every new writer is "What should I write about."
Unfortunately, there is no universal answer to that question. There was an old sitcom, "Bridget Loves Bernie," in which one of the characters is told by his publisher that he should write only about something he knows a lot about. So, he goes and writes a book about love been two Eskimos, which someone else publishes and which wins him rave reviews. They must not have noticed that the author lived on the Upper East Side and not in Alaska.
The list of things to write about is easier to narrow down than it is to specify. It is a good idea to write about something organic to you, not to some other authors. The less your mind and intelligence come into play when choosing a topic, the better. The "intelligent" thing to do is to look and see what is selling, then to mimic that in your own writing. Now, sometimes that does work, such as with the recent craze in Vampire books. However, your chances of crafting an inferior, derivative product go way up when you do that, never mind the competition you will have in getting people to read your vampire novel as opposed to all the other ones with equally catchy titles and splashy covers.
I suggest that you go your own way. If you like science fiction or romance novels, write that. Find something that grabs you and stick with it. This will not only help you create something that you can relate to, but you likely will wind up getting it done faster (for the same level of quality). Also, it helps to put your best foot forward first when you are a new author. Get a quality product out that comes from your heart and soul, and only then, if you must, do something only because you think it will sell. It will help your self confidence and maybe attract a few readers of your first work who will stick with you. That is how you build a following. There is always time to become a hack later.
Quality pays in the end. If you stick with that as your guiding theme, you really can't go wrong. Build a reputation and do your best, then try to gather your audience. If you like it, chances are excellent that there is an audience for it. Your next job, after completing and publishing the book, is to find that audience. And, believe me, you can do that. In the final analysis, you will feel so much more satisfied if you develop an audience of like-minded people rather than those to whom you are simply pandering.