1. Shutterstock. The "King of the hill" in microstock world, I just can say that it bring me about 50% of all my stock income. You will need pass exam by sending them 10 your best images. If shutterstock approve 7 or more of them, congratilations, you in! Also, you will need provide your international passport. Minimum image size: 4 mega pixels (MP).
2. Istock. Also very big RF-photobank, founded in 2000 year. Has an exams, You'll need to provide 3 images for it. Accepts photos, 3d renders, any rasters, vectors, music, flash templates. Requires some document of your own (passport, id, etc). Minimum image size: 2MP.
3. Dreamstime. Good Romanian photobank, with no exams. Often keeps second-third place in sales. Has a rating for each image, which depends on sales count. The more sales - the more rating, and more earning for one sale. Minimum image size: 3MP.
2. Istock. Also very big RF-photobank, founded in 2000 year. Has an exams, You'll need to provide 3 images for it. Accepts photos, 3d renders, any rasters, vectors, music, flash templates. Requires some document of your own (passport, id, etc). Minimum image size: 2MP.
3. Dreamstime. Good Romanian photobank, with no exams. Often keeps second-third place in sales. Has a rating for each image, which depends on sales count. The more sales - the more rating, and more earning for one sale. Minimum image size: 3MP.
4. Fotolia. Also good and profitable microstock. No exams needed. Sells regularly, in case of good portfolio. Minimum image size: 4MP.
5. Depositphotos. Young and promising photobank. Has an exam, but not so exacting, as his older competitors ). You'll need provide 5 work. Minimum image size: 4MP.
6. 123RF. Not so well-publicized, but good microstock. Requires passport, and pass exam of 10 images. Minimum image size: 4MP.
7. Pixtastock. Japanese microstock. Average or low in rating, but sometimes do some good sales. Minimum image size: about 0.5MP.
There are a lot of other photobanks, but these are the best, the most profitable and will bring you most of your earnings. All you need - go through links and register your account, it is absolutely free!
All links will bring you to registration as my referrals, this will not affect you in any manner, but my referrals can write me personally (via personal messages) if they need any help on microstocks.
Remember, all images you produce, must be absolutely your creation, even if you redraw or copy some photo, this must be your photo also. Don't copy any work of somebody. Yes, you can browse microstocks popular works, but if you want to imitate some work, do it in your own manner, try to go deeper, look on it at another angle, with new thoughts and produce your own unique content!
Find your niche. For example, I know one very successful stocker, who makes very good sales, and creates only images of fire. All of them very quality and diverse: fire letters, musical instruments in fire, sport equipment in fire, and so on.
Good luck!