From what I've seen, you are most likely to make a steady income from weddings, portraits, and commercial studio work, like product shots. Your best way to get started is to assist another photographer to learn the ropes.
I've sold several pieces of my own personal work over the years, but it's definitely not a steady income. I might make $150-$300 from an image hanging in a show or gallery that sells, but the sales are few and far between. I haven't done many weddings, but my first two made more than all of my art print sales combined.
If you want to make money as a photographer, it will definitely take an investment; mostly of time. You have to get good to expect to sell anything, especially now when it's even easier to grab yourself a snapshot. You need to stand out from the multitude. Just "nice" won't sell as an art print. People might like to look at it, but not enough to spend money on it. And for commercial work, you need to know your stuff to stand out from everyone else looking for a job.
This isn't to discourage anyone from making money. I think you can do it with some dedication and time. But I think people shouldn't put effort into it until they've been photographing for a while. Put the energy into the photography instead. I think you'll get better returns.