There is a LOT of information regarding the questions you have inside of Models Only; Before getting your hopes up (or dashed, for that matter), definitely get verified as a model and ask your questions inside MO.
Let me put it this way: There are girls who make Top 20 in their first month. There are also girls who take a year or more to reach the Top 100. And there are FAR more models who never reach higher than the 200s... or even lower. Just logging in will not ensure success, especially on a site like MFC, and there is a good deal of luck involved in addition to all the hard work. It's not a matter of just logging in and getting tipped, it's much more difficult, stressful, and time-consuming than that. The top models may make it look easy, but that doesn't mean that it is.
Definitely get model status, and then ask these questions inside of MO for more detailed answers.
Edit to add: Just because you're not ranked in the top 100 doesn't mean that you won't be making a good living off of being a full-time cam model. I've been doing it full-time for a little over 1 1/2 years now, and make more than enough to live off of, put money in savings, save for taxes, and afford a few little luxuries. Obviously I don't make 25k a month, but I don't really NEED 25k a month. I could push for it and get there if I wanted to (I've gotten close before, back when I was pulling ridiculous hours on MFC), but I don't want to sacrifice my life outside of camming for the sake of money. Now I work 2-3 hours 5 days a week on SM and make enough to be more than comfortable. It's all about finding a work/life balance with camming.