How can I come across as intelligent and confident?

People tend to view me as dumb and illiterate, even though I don't feel this way, and don't know what I am doing to radiate stupidity. I was raised to be humble, polite and am also very quiet. Could this be why people immediately ume I don't know very much? The other day, for example, the hostess of a restaurant took one look at me and began speaking to me not only in spanish (I am hispanic, but do not speak spanish), but also in a very condescending tone. Does anyone have any tips on how to look more confident and intelligent? I'm tired of being treated like a second cl citizen.