what are sororities looking for in upperclassmen?

Ok I really want to rush as a freshman but I know I need to get my grades up and rush as a sophomore. I'm just not sure what sororities look for in upperclassmen? Obviously they look at your grades but what else are they looking for? Do they look at campus involvement? Or do they just make sure you have the right grades and that you fit in with the sorority? I'm really lost so please help!