Like you're finding out from everyone's replies, all schools conduct Greek life and housing differently.
At my school, you go through rush your first week (some schools start rush the week before school begins, some hold rush in the spring) of school. During rush week, the girls will get to meet members of different sororites and get an idea of who they think they'd like to be sisters with, and the sorority sisters will narrow down who they want to join their sorority. Sororities will "drop" girls throughout the week (say no) and at the end of the week, girls may (or may not, depending on how many girls rush and how many sorority spots there are available) receive bids to one or a few sororities and will "pledge" to the sorority they want to join.
The pledge period is sort of an initiation (usually not much hazing in sororities... fraternities at my school, that's a different story) which takes most of the first semester. They aren't allowed to live in sorority housing until they are members (2nd semester) and that is based on availability.
At my school, you must live on campus during your freshman year (or at home with your parents if you house is within 20 miles, I think). It's a good way to meet other students and get to know people who are and are not involved in Greek life.
Also, it's important to note that Greek life is rather expensive (prepare to spend about $1500 a year on dues), and the rush week can be terribly judgemental. Because they only have a week to get to know 500+ girls, a lot of evaluation is based on appearance... well dressed, natural beauty, etc. At my school, being a legacy (mother in sorority) will only get you through the first round, then you're on your own. It's better to have a sister who is in or was recently in the sorority... then you're almost a definite because you have a strong pull from inside.