A class valedictorian presumably has had a top scholastic record and probably done a lot of homework and read a lot of assignments. Somewhere in there, there should be some kind of inspiration. What has moved you? What has affected you deeply? What is the best of your experience? What are the hopes and dreams and aspirations of your class?
This is your one big opportunity to address your classmates (and other attendees--look up some samples to see the right protocol for order of mention--administrators, faculty, parents, honored guests, etc., and fellow members of the class of 2008) with your own personal thoughts on what your school experience has meant to you and what you see looking forward for your fellow students. Ask your class advisor, your own advisor, or your favorite teacher for some guidance on how to structure your speech, and then you can pretty much say what you want.