Determine your GPA. This is usually given with your school grade reports. GPA changes with each class taken. So if you will be taking more classes, use your best estimate of how you think your grades will end up when you finish current courses.
Compare your GPA to the equivalent UAI score. A GPA of 6 to 6.5 would have a UAI of 91 to 94. A GPA of 5 to 5.99 would have a UAI of 88 to 91. A GPA of 4 to 4.99 would have a UAI of 84 to 88. A GPA of 3.75 to 3.99 would have a UAI of 72.
Contact the University you want to attend. Most Universities have websites with contact information for the admissions office. The GPA to UAI conversion varies from school to school, so to get the exact conversion your school will use contact admissions.