Ditto to everything that was said, you've gotten some very wise advice that you should definitely take to heart.
I'd also like to add that you need to
use your resources. The internet provides you with a plethora of information so seize it.
From the way you worded your post it sounds like you are primarily interested in Panhellenic Sororities -go here:
http://www.npcwomen.org/ and check out the sororities that are NPC members, specifically the ones on your campus.
Go find the Greek Life page for your school and look at the websites for the chapters if they have them. Do your homework! Think about what you're looking for in a sorority and then try and find it on a National level. That's the "top tier" you should be worried about.
Obviously there aren't just official websites, there are places like GC that also have a huge amount of information. Take the things that are said on these websites with a grain of salt, but again they can be extremely useful. One of the best things that I ever did was to just come on to GC and start to read. I read here and there, tons of different greek-related threads, but slowly over time I immersed myself in the information and having that knowledge helped so much! One of the big things that it did was to prepare me for anything. There's no real way to know how intense your rush experience/your campus's specific Greek culture will be until you experience it for yourself. However, reading about everything from all different perspectives will ensure that you are prepared for just about anything, or at least more than some others around you. For example, people start yelling and banging on doors during your first recruitment party but you've read about things like that before so you're not completely thrown for a loop and so you're able to hold an actual conversation...you get the picture. Build up your knowledge about the Greek system in general and you won't be as nervous.
As for the not knowing anyone, you need to put yourself out there! I don't mean on like the social/party scene, but rather in your school community. Don't make excuses, make it happen! If you're in class or in line with a girl in letters strike up a conversation with them, not necessarily about their sorority. Just let them get to know you a little bit! I'm know that the reason I got my bid was very very much to do with a girl that I met in my English class less than a week before rush. We'd made casual "class friends", I knew that she was in Kappa Delta and the day before rush started I asked her how she liked being in her sorority. I told her I was sort of interested and she was instantly on me to do it, saying I had no idea how awesome it would be. She was the only person that I knew in the chapter but she fought tooth and nail to get me on our bid list over other girls who "knew more people" Granted we don't have an extremely competitive rush overall, but it was a pledge class of only six with
a lot of girls participating. The bottom line is that you don't have to be best friends with everyone, you do have to make a good, strong impression though.
Be memorable and in a good way. One of the things that my father always taught me to do was to have a good handshake. You'll be meeting a lot of people, maybe no one will notice it -but maybe someone will. During my second party at KD I met the only girl in our chapter who is a legacy and she commented on me having "the first real handshake of the day." Months later in her senior passdowns she gave me something to remember her by and said "I can still remember you shaking my hand for real and I walked away thinking 'That's the type of girl I want to be my sister.'" So like I said, be memorable.
So this is already way to long, I apologize. I was going to PM you but I thought it was good advice and who know who may read it and find it useful.
Hope I helped and didn't bore you to death (really I'm just stalling from paper writing lol) Best of luck to you in your search for sisterhood! And let us know how it goes...we alllll love a good rush thread here on GC