This is not quite 'Stock Photography 101' as I cannot claim to be anywhere near an expert. However, I'm happy to share some key things on what I have learned on the way to where I am now.
There are numerous stock websites on the internet to which you can upload photography and offer them for sale by download by their clients. Your commission on the sale can range from pennies to hundreds of dollars per usage. Some sites are no doubt better than others in terms of traffic and rewards.
There are discussions, arguments, possibly even boycott movements, etc. regarding the merits of letting your images go for pennies through microstock versus holding out for a 'respectable' professional fee. There is a sense that microstock undermines the work of professionals. I won't disagree. I just try to avoid these debates and get on with it. The internet environment is a rapidly changing one, as is the photography environment. I just try to play on the field that is out there the best way I can, and what I did yesterday is not what I do today, and I don't expect that what I do today will necessarily work on the playing field of tomorrow. However, having said that, I'm not partial to giving my photography away for nothing - i.e. simply an acknowledgment/photo credit. I have done it, but rarely, and in selected cases. But that is off topic.
As a 'starter' in stock photography it is probably the easiest to become accepted to be a player in the microstock sphere. I participate in a few agencies, and of those few I would recommend Shutterstock and Dreamstime. These sites sell images at very low cost to the buyer and consequently your commission is low - but sometimes the volume can be relatively high. The $0.25 to $0.50 you earn per download can add up over time. There is a large investment in time required to upload, title, keyword and categorize each photo, but once done they are there to stay and can continue to earn for you into the future. If it turns out that you have an 'eye' for stock shots and have fairly good technical abilities, you can earn a few thousand dollars a year from an equal number of uploads. Some photographers do much better, some don't do well at all. I'm just speaking from my perspective and you won't know unless you give it a try. You probably won't make a living at it, but it can be a supplement or at least be a way make a down payment on the upgrade in camera gear you would like or a camera quality that your growing addiction to stock may dictate (yes it can become addictive). Note that some stock sites won't let you become a player there unless you can produce a certain quality/size of image and they may even go as far to exclude camera types. You will find out soon enough when you try to register. (I still use a Nikon D70s, a good camera but not good enough for some stock sites). Others, even Getty Images, will accept good images from my Canon PowerShot A570IS which I have used for kite aerial photography. If you encounter an equipment limitation at a site, you can only move on and wait for your camera upgrade.
With most stock sites, you will be required to submit several samples of your work for approval. Take this seriously. Some sites won't let you reapply for a long time after you have failed. Read their image submission guidelines and quality control FAQs religiously before you submit. If you are like me you will learn a lot about image quality and it is best to take some time to learn what they are looking for before you go through your files to make your submission selection. Once accepted you can risk some of your borderline shots which may be rejected. But in your first submission, just send your best. Be prepared also that once you are in, the acceptance rate can be very low. Keep trying to learn what they want and what your mistakes are. Don't get hot under the collar and start firing off in their forums about "what's wrong with this one". Just move on to the next image, or try the rejected image at another stock site if it is that good a shot. Read the FAQs and learn. And don't submit images with people in them unless you have a model release (also in their FAQs).
Before diving into microstock too quickly though, weigh the pros of being an "exclusive" photographer with one agency such as iStockphoto. www.istockphoto.com is the third site I would recommend but personally I found it harder to get in the door with them. My submissions failed the first two times and then I simply gave up for about 12 months. Now that I'm in I see that the rewards are slightly higher than the previous two mentioned and their exclusivity option is tempting. There are stories of young photographers (talented mind you) earning 6 figure incomes in one year through iStockphoto. (Dream on I say). Basically they ask that you make them your only stock agency in exchange for higher commissions. However, in the case of iStockphoto, becoming an exclusive with them isn't an option until you have had 250 downloads/sales. So here's the dilemma. In the time it takes to be first accepted, and then get to the 250 download point, how much time and energy should you invest in stock sales elsewhere. You will have to chop up your accounts at other stock agencies if you intend to become an exclusive photographer at iStockphoto. I have not yet reached the 250 mark. I learned about the exclusive option after I was well into uploading photos elsewhere. So while I ponder, I'm less ambitious about uploading to my other stock sites for penny rewards. iStockphoto's market strategy is obviously working as they have succeeded to have me withhold new submissions from the other stock sites while I ferment the options. I've put myself on hold with Shutterstock and Dreamstime until I see how my iStockphoto experience matures.
The one exception to iStockphoto's exclusivity arrangement is Getty Images. There is an affiliation between the two companies which I don't fully understand, but if you've made it into Getty Images, you can continue there and take full advantage of iStockphoto exclusive arrangements.
Until about 18 months ago, Getty Images seemed to the uninformed like myself, to be a domain of the expert photographers; an enviable and inaccessible fortress of the professional. Whether I was right or wrong doesn't matter, because a change occurred at that time. Getty made an arrangement with Flickr.com whereby Getty could glean through the ranks of Flickr and select images and photographers to join and submit to a special 'creative' line of Getty Images. I was fortunate to be one of those Flickrites 'found' by Getty. There were several glitches and growing pains in the arrangement but most of those that made sales recognized that there was merit to being in the program. Any image selected has to be a Getty exclusive but it doesn't mean that you have to stop selling other stock through your other agencies. You are just not permitted to sell elsewhere the images you sell through Getty. It has to be a new image that you have not sold anywhere previously. And you cannot sell 'similars' elsewhere; those being images taken at the same session of the same subject or sufficiently similar from another session to be seen as related. In a nutshell, they don't want two buyers to have a similar image that will compete with each other in the marketplace. A downside of being in the Flickr/Getty process is the long review time after you have submitted an image and the long time it may take to get a sale on a particular image. The upside is that sales can sometimes come with significant commission (by this I mean several dollars to several hundred dollars from one sale). Initially it was a shame that you had to be 'discovered' by Getty on Flickr. I have friends who wondered how to get in front of their face .... there are a lot of images on Flickr and you can be easily overlooked as my friends were. Fortunately there is now a mechanism to put your images in a queue for Getty reviewers to 'consider' so that you may be discovered more easily. You can read all about it yourself here: Getty Images Call For Artists. I can only guess that it may be months and months before photographers on the newbie list get reviewed and, if fortunate, invited to participate. Start the ball rolling there soon just to get the door open. Good luck with that and be patient. Remember to only use an image that you will not submit elsewhere. Also remember that Getty employs people to proactively sell imagery out of their database. They match images with clients and charge higher fees from their clients for that service. So be patient for what will hopefully will be larger rewards.
As small or large as sales may be, stock photography is a good way to force yourself to improve your skills. The agency websites give you guidelines, FAQs and discussion forums where you can learn the trade and make friends. Plus you sometimes get 'limited' review of some of your images (other times you get form responses that seem totally inadequate). But for someone that is starting , I think one important thing to take away from stock photography is that somebody out there liked one of your images enough to buy it. Even if they only spent a few dollars and you only received a few cents. Satisfaction comes with knowing you are being appreciated and on the right track with your photography.
Comments and links to other advice, resources and useful stock photography sites welcomed.