Liz -
Hey sweetie - so your whole life is going to change in a little over a month and you've got butterflies in the tummy? Congrats, you are normal!
Just to reiterate what everyone else has said - meetings will always be the same time each week and your non-Greek friends will be aware of that - just like if they have a meeting for their org the same time every week. If you've been involved in a ton of things in high school, you might not be able to involved in as many different activities but you will get more out of the ones you are involved with. I don't think there was a single sister in my chapter who didn't have non-Greek friends or wasn't involved in extracurriculars.
I'm glad to see you're thinking all this through in advance and not just getting caught up in the excitement - it shows a lot of maturity. Good luck, and I know you'll have fun at rush!