Hey!
Basically, you sign up directly on a camsite, send them the documents they need, and once you're approved, you can start camming. MyFreeCams, Streamate and Chaturbate are three of the most popular camsites, and the vast majority of girls on this forum work on those sites.
But I would suggest that before you sign up anywhere, you first watch a lot of different camgirls and read the information you can find on this forum! ACF is a really great source of advice for new and established models. By reading the forum, you can also figure out which camsite might be best for you - though there is no shame in trying one and then signing up on a different one later, if the first one you try was not the right fit for you.
After you've signed up on a camsite, you can become a verified model on ACF, which will allow you to enter the Models Only section full of even more advice.
I can't give you any information on what girls on other camsites make on average, but on MFC, the average amount of money girls make is around $20 per hour. However, there are girls whose average hourly income is closer to minimum wage, and there are girls who make several hundreds of $ per hour.
There's a lot of earning potential on MyFreeCams but there's also a LOT of work and luck involved. The kind of money you make depends mostly on how much work and creativity you're willing to put into camming, and how seriously you take it.
edit:
Amber was faster...
But yeah, definitely check out the camgirl wiki as well. Lots of information in a condensed form.