I suggest you search the internet for the microstock vs macrostock debate. Microstock can be fine for someone, but is probably a waste of time for most. With microstock the recipe for success is producing material than can sell many, many times (office presentations, blogs, small publishing, newspaper use microstock as well). The typical commission that microstock agencies take is 70% or 80%, meaning the photographer only gets 20%, or 30% of gross revenues. You'll have to do all the keywording by yourself.
Macrostock agencies tend to cater to different clients (newspapers and magazines, but also books, and especially advertising, there certainly is some overlapping with the micro market, but not that much) some of them specialize in very narrow market niches, some other are more generalists. You'll have to wait many months to see results, which are often reported quarterly. The client might buy a picture for a book publication, pay only two months after the book is printed, and your agency will pay you only one or two months after receiving the money. But the share is typically much more honest (agencies take typically 50% of gross sales, some of them do the keywording for you) and the longer-term rewards are typically much better, unless you produce exactly the kind of material which sells best in the microstock market.
The big dogs in macrostock are Getty Images and Corbis. Very difficult to become a proper Getty or Corbis contributor. Getty gets an 80% commission but tend to sell a lot, especially in the commercial (advertisement) arena.
Other names that you should look into: Masterfile, Photolibrary, Alamy at the very least. There are literally hundreds, or thousands, or macrostock agencies. Good research.
Alamy can be singled out for some original features: very open to anybody to propose material; selection is basically made "per picture" not "per photographer"; picture selection is made solely on technical ground, not on saleability; quality controls are on a small percentage of randomly chosen images; when an image fails a quality control, all images waiting for approval in the queue are automatically rejected (so called "zero tolerance" policy).
Another feature of Alamy is that a complex ranking system discourages "keyword-spamming".
If you want to test the macrostock agencies waters, I suggest Alamy as your first port of call. Alamy is one of the main agencies for "secondary editorial" material, that is, material which is not used for advertisements and which is not news of the day.
Remember the stock market has its own needs, pictures sell if and when they satisfy the needs of a picture researcher, to illustrate a book, an article in a magazine etc. There's almost no call for "artsy" pictures, "personal vision", "originality", "experiment" or "poetry". Nobody cares about who you are and what your "artist statement" might be. Meat is much more important than smoke. Good old photography is what sells. And I LIKE that.