Good luck, MTSUGURL! I loved my time in the League, and am now proudly a sustaining member. People who are not Greek or League members roll their eyes when I say this, but I have met my dearest, truest friends in life through my participation in those two women's organizations. Being Greek, I'm sure you understand!
Leagues are different all over. Some you have to be recommended in triplicate and vetted for six months before they offer you "provisional" membership (like a pledgeship). Others are less selective. Either way, it's an honor to serve! I wish you all the best.
As for what to wear, I'd go with a smart pair of dressy pants, maybe a bit of a heel, and a sweater or blouse. Twin set?
Or a cute skirt with boots. Something you'd wear to work in an office, but a step down from a suit. That way you'll be fine if the majority are there in suits straight from work, but you won't be dramatically overdressed if it's more casual.
Keep us posted!