Wherever you go to school, recruitment gets progressively dressier with each round.
I'd first look at what your school recommends for attire for each day. Call the office of Greek Life, or check out their web site (sometimes they feature What To Wear slideshows).
Next, look through your closet and see what you may already have that is appropriate. Then make a list of whatever items you need. Using your university's What To Wear recommendations as your guide, buy any remaining clothing and shoes.
Break in your shoes-- you will be walking and standing a lot. Also remember to keep any skirt lengths modest-- you may be sitting on the floor at some of these events.
Keep cleavage, tummy, thighs and butts tucked in-- sorority recruitment isn't the venue to "let it all hang out"