I advise you to keep it short and simple. Put every word you want to say in large type on small numbered cards. If you think there should be a pause, be sure to note that on your cards. This will be just like Winston Churchill did his speeches, but be sure and DO NOT read "Pause for applause".
Here is a tried and true graduation speech. Start with a gracious welcome to the people who have attended -- include school board members if they are there. Thank the people who should be thanked, including the musicians. Say how honored you are to have been chosen to represent your class. Describe some of the highlights of your four years, especially ones that were particularly meaningful or taught you and your classmates something.
Note how young you were when you entered high school, how much it was like a beginning to adulthood. Now you and your classmates are leaving high school and entering the real world. You and they will be going in many different directions but every one of you has the opportunity to make the world a better place. Describe your hopes for how the world could be a better place.
End with a joke; for example, remember when you wake up next Monday, DON'T go back to the school.
It's also nice to end with an "I love you Mom - I love you Dad!" especially if you are looking directly where they are sitting.
Congratulations on your accomplishments, your graduation and your chance to speak for the whole class. It truly is an honor.
Good Luck!