If you've met one Autistic person, you've met one Autistic person.
I feel like that's all I should have to say on that. If someone tells you that they're Autistic, believe them. Don't say, "but you don't seem like my neighbor's five-year-old son," or something ridiculous like that. Don't say, "are you sure?" or "do you have a real diagnosis?" The Autistic community gladly welcomes the self-diagnosed, because we understand that getting a diagnosis is just sometimes not possible or practical.
Also, functioning labels* are lame. Don't use them. No one wants to be called "low functioning" and what does it really mean to be high functioning? We have uneven skill levels. Autistics are often "high functioning" at one thing and "low functioning" at other things. It also depends on the day, or time or day, or whether or not we're hungry or tired, or, or, or...(you get the idea). Read this. It's good and explains the Autistic Spectrum better than I can.
Asperger's is a form of "high-functioning" Autism. (That's what Autism "professionals" say anyway.) The DSM-5 removed Asperger's as a separate diagnosis and instead put it all under the umbrella of Autism Spectrum Disorder. I still use to the term Asperger's because people know what it is, but please know that Aspie supremacy drives me crazy and won't be tolerated here.
Please be aware that just because someone with "high functioning" Autism many look "normal" and can even act pretty "normal", this does not mean that they are "normal". "HF" Autistics are still Autistic, and still need support and understanding. Just because there are a lot of things I can do, there are many things that are incredibly difficult for me, and some things that I am simply unable to do. Don't judge by appearances, and also remember that every Autistic person's experience is different.
Bottom line: when you interact with an Autistic person, just don't be a jerk. Don't assume, don't stereotype, and for the love of all things, don't judge.
*I'm going to refer to them here for clarification purposes and because they're still used in the professional communities (much to my chagrin).