If you Google "Stock Photography" you will find a million of them (more or less). Personally, I'm not a big fan of them, and don't as a rule recommend this for several reasons:
1. With the prevelance of good quality P&S and DSLRs available, a lot of people are doing this. Stock sites are flooded with images, most of which aren't all that good, but are cheap. The result is that photographers are getting paid less for their images, and the quality of images used in many cases is less because quite simply, why pay top dollar for a good image when you can pay low dollar for a mediocre image?
2. Unless you're in the top .1 %, you can except to get from a few cents to a couple of dollars if one of your images is used. A lot of stock sites, especially the less reputable ones require that you give up all rights to an image when you post it on their site. That means if it does get sold to a big buyer, instead of potentially hundreds or thousands, you'll get a few dollars.
I would suggest start by trying to sell your images locally; print a few hundred of your best (in varying sizes) and set up a booth at craft fairs, see what sells, listen to comments from people. It will take a little bit of an initial investment, but it will give you a much better idea of where you stand, and what your chances of making money as a photographer are.
If you do want to deal with a stock site, do your research, find out which ones have the most stringent screening process, Google for reviews and comments about each, find out which allow you to retain at least some control of submitted images, and
most of all: Good luck!
Just my $00.02 worth - your milage may vary.
~John