Corina,
Just wanted to re-iterate that we are all behind you! Again, you said that you have 18 girls who were really interested and I am sure that by word of mouth, that number will grow. Do you have a friend who is as keen as you are, who can help you out a bit (don't neglect your school work....a good GPA is important to sororities)
18 girls seems like a great start because I had a look at the 1999-2000 Gamma Phi Beta composite at UCSC (it is
here and from the composite, there only appears to be 25 members. Even if, say 10 girls missed that day, that is still a chapter of 35 women. So, you are off to a great start if you have 18 serious interests and if that number is growing!