If we are talking microstock, then:
Register with your ID, if it is needed, pass the exams on iStock and Shutterstock. Upload, wait for inspection. Hope they sell
Worth mentioning are iStock, Shutterstock, Fotolia, Dreamstime. (imho)
If you want to join Getty - you need to send them a letter with a link to your finest pics and let them decide, if they want you in, or not.
Alamy is free for all, as I remember, but not much sales there.
You also can try some production agencies, but I have no experience with them.