"I have autism"

uh..don't open with this..

-The perception of autism has been ruined by being popular. Since very few of the nonphysical autism symptoms are unique to autism, you can maybe try showing the bits that are normal first and explain at some point when they notice differences? I myself did not know there were physical symptoms until I read the Wikipedia entry.

What's a good way to phrase this so a person knows I'm not being snarky?

-A large portion of comedy is just timing. As such inappropriate timing will bomb really hard. It's also quite common for a person's personality to not match their physical appearance or language use. If you mention things early and often, and not after issues arise, it should be okay. Also, mention if you have trouble with tones and inflections. (This made me think of the Johnny Carson tomahawk bit, where he kept his mouth shut for a solid half minute before saying his line)

But isn't someone informing them also how people learn?

-There are multiple ways to learn, and that's actually one of the less common ones, especially if it's just a flat "don't" without an actual answer. The most common method being things exploding in your face.

-It's rather easy to learn from prior knowledge, but all of human knowledge had to be discovered at some point, which means somebody had to fuck it up repeatedly to get there, because they sure as hell didn't get perfect completion on the first try.