It amuses me to think that any school administrator in his/her right mind would ask Holden to give a graduation speech. However, I can imagine that if this very strange choice for a graduation speaker were to occur, Holden would have some pretty sarcastic and negative things to say, i.e.:
Hey, you morons! This place, Pencey Prep, is full of phonies. I don't know why they asked me to speak, since I flunked out of this place four times, but what the hey. I see some of my so-called friends sitting in the audience. There's "old Robert Ackley" over there, still covered with zits. And, hey, "Old Stradlater" - dude! I never did write that paper for you so you could go out with Jane. But I'm not sorry.
"If you really want to hear about it," ............
You get the idea. Go through the novel and pick out some of Holden's more colorful assessments of his time at Pencey and his evaluations of his "friends" there, and you can put together a graduation speech.
Another approach would be, assume that Holden actually is healed at the end of the novel (although we don't really know this), and have him give a graduation speech based upon the lessons he has learned from being a messed-up teenager.
See the analysis here on enotes for help.