Whether photography is a hobby or a job, stock photography is a great way to get some extra cash. Being a stock photographer, however, is not as easy as it sounds.
People are often too impatient. If your photographs aren't accepted right away or you aren't making any money don't be discouraged. Stock photography is something that requires patience.
People quit after their photos aren't accepted. Just because your photos aren't accepted doesn't mean that your pictures aren't good. Many times, stock photography websites will deny images because there are too many pictures that are like yours. Try to take unique pictures, if you do, it is less likely that this will happen.
People do not use noise-reduction software on their images. Some are expensive, but they are a crucial part of stock photographs. A simple web search will bring up many good noise-reduction softwares.
People aren't critical enough of themselves. Inspect every single spot on the pictures that you are going to try to sell. If you find a problem, fix it with your photo editing software. If you don't find these problems, the stock website will, and may reject your images.
Try to find somewhat unique material. If your photo is very similar to another then you are less likely to sell it. There are tons of stock photos out there. The key is to make your pictures different.
Edit your pictures a lot, if you don't then pretty much every other stock photographer will be a step ahead of you.
Think about who would buy your photo? Where would the photo be used? If you can't figure out where, then it's probably not the best stock photo.
Stay active on stock photography forums, they will likely help you to become a better stock photographer.
Things to Keep in Mind:
You aren't going to become a millionaire overnight. Stock photography is a good way to maybe make a couple thousand dollars extra every year. It's probably not the best idea to count on stock photography as a main source of income.
It's not all about the camera. It's about the photographer behind the camera. Even if you don't have the most expensive, highest quality camera, you can still be a stock photographer! Many small, less expensive cameras have tons of features that you might not know about. Reading through the manual for your camera is often very helpful to find some of these incredible features.
Have a good time! Don't treat it as a job, treat it as a hobby. You will definitely enjoy yourself much more if you do.