Your first step should be to stop by your student activities office or the advisor of current student organizations. Many universities now have rules against having local organizations and all student organizations (Greek and non) must have a national/state affiliation outstide of the institution (for liability reasons).
For expanding, you will want to contact your NPC area advisor - if you go to
www.npcwomen.org (which I can imagine you're already familiar with
), you can get the contact for your area and she will be able to help you from there.
A word of warning, if you start an interest group (and likely that will be something you'll need to do for your institution to take you seriously), do not focus on being a part of any one chapter. What will happen when you open for expansion is that you can invite up to three NPC groups to present to you and you will choose one of the three to be your new colony - it's a very interesting process and likely well worth the work! Good luck to you!