Decide on an appropriate theme for the function, or if a theme is appropriate. Are you planning a cocktail party, a formal event, a community service clean up or a fund-raising event? This will dictate whether or not a theme is called for. For example, sororities would likely select a theme for a cocktail party but probably not a fundraiser.
Create invitations and a guest list for the function and mail them out with enough time for people to confirm their attendance. An RSVP invitation will help budget the event so there is an accurate estimation of how much supplies are needed.
Allocate some function-appropriate entertainment or hosting services. If you are throwing a formal, you will want a DJ or a live band. If you are throwing a community service clean up, you may want to book a motivational speaker to tell participants the importance of their work.
Get food. The promise of a bite to eat will draw people in who would ordinarily not come. Decide if your sorority would benefit from some light snacks or full catering services.
Decorate the venue for your sorority function. Think about what kinds of adornments would compliments your event's theme and purchase them. Then, recruit your sisters to help you with the leg work.