First things first. In most cases, a sorority is a social and service organization for collegiate women. If you're interested in joining a sorority, you're going to be involved with community service activities, social events with the members (and members of other like organizations), etc.
There are many kinds of sororities, many of which have banded together to form conferences (kind of like in college football where there are "conferences" that play against one another and follow similiar rules). The most commonly known sorority conferences in the USA are:
The National Panhellenic Conference
www.npcwomen.org
The National Pan-Hellenic Council
http://www.nphchq.org/
There are also a large number of Cultural Interest Sororities (Asian, Lesbian, Christian, Multi-Cultural, etc.)
http://en.wikipedia.org/wiki/Cultura...and_sororities
Each conference or group has its own methods of membership recruitment and membership requirements. Check them out by organization and conference and see which ones may be more suited to your interests.
In order to be considered for membership in of one of these organizations, you must be a student at a university with an existing chapter of the organization currently exists. (there are some exceptions, but this is one way to get started.)