You do not have to concentrate on one subject as long as you know what you are writing about. If you already know a subject well (something which you have known since childhood, or perhaps you worked in that area, etc.). It can be work, a hobby, or something which you research well before you write about it.
For instance, I began writing about birds because I have been into them for many years - watching them, feeding them, etc. I also began trying other areas to see how they felt to me when I wrote them plus what types of responses I received.