Sororities will be looking at your GPA from the first semester. I honestly don't know if we even get information regarding your HS grades.
The minimum GPA is 2.5, but many sororities have their own minimum GPA. I would recommend that you shoot to get a 3.3 or above. Girls with a 3.5-4.0 probably have the best chances, though you'll be able to find your home even if you don't make that high of a GPA. However, GPA does make a big difference!!! SMU has a lot of resources to make getting a high GPA your first semester possible -- check out the learning center for tutoring, and go to your professors' office hours!!!
PM me if you have any questions about recruitment or SMU in general. I'll be happy to answer any questions you have!!!