Just explain your condition. If you know you have it for sure, you can tell him you have done your research and found out you have hyperhydrosis. He might examine you, for example he might have a look at your hands if you are sweating. Depending on what kind of HH you have, he might want to start you off with a topical medication like Drysol.
Don't be nervous, but even if you are, it doesn't matter. Just explain your condition and see what he recommends. Most doctors are not experts on HH so you might know more about it than they do.