Congratulations on being selected for such an honor.
Well, let's start with the definition of a valeditory speech. It is usually an expression of farewell or leave-taking. You could start with that and recall some of the highlights of the years with your graduating class and move on to cite achievements by your classmates and what the future holds for your peers.
Think of it as a review of past years together as well as a pep talk to your peers as you move on and go your separate ways.
Do a search on www.google.com
for highschool valedictory speeches to see examples of what others have written in the past.