I have been investigating how stock photography agencies work and carefully reading their terms and conditions for signing up with them. I have concluded that the best stock photo agency markets to sell your images on are what are known as "microstock on-line photo agencies".

What are the advantages to using Microstock over bigger Stock agencies?

First, they accept submissions from a much wider range of photographic skill levels.
Second, they pay beter commissions.
Third, they have better "exit" options if you decide to end your participation.
Fourth, they give you the opportunity to make a name for yourself before you
approach the bigger agencies.

Here is a list of seven "Microstock" agencies:

iStockphoto
Shutterstock
Dreamstime
Fotolia
StockXpert
Crestock
123RF

I suggest you visit each of their websites and very carefully read over their terms and conditions. Most of them will want exclusive, world-wide rights to your images; so you have to be selective of what you give them - you won't be able to market those images elsewhere once you commit. Compare their payouts, image resolution parameters, and other important benchmarks. Some agencies are stricter than others.

Also, browse through their image catalogs to acquaint yourself with the type of images they sell. If you want to enter the ranks of professional photography, this is a great place to begin because your start-up costs are very low - a good digital camera and your time.