pronunciation - "A" or "an" with words beginning with the letter H

I am wondering when it's correct to use a/an with words beginning with the letter h. For example:

an historian, a history.

I remember reading somewhere that the h is sometimes silent, in which case it's an, and when the h is pronounced, it's a. But then I also remember reading that it depends on which syllable is stressed. And I also think I read somewhere that it might differ between British and American English.

So how do I know when to use a and when to use an with a word beginning with the letter h?