You pronounce the word “herb” as “erb” – with a silent H. This is because of the fact that herb originated from the French word herbe which also originated from the Latin word herba. Because the word herb was only borrowed from the French, the world followed how they normally pronounce word with letter H on it (although there is always an exception to this rule) as if the letter H does not exist at all.
However, there was a time in the United Kingdom wherein people started to pronounce “erb” as “herb” – take note: They started pronouncing the letter H because they thought that that is how they should be actually pronounced. In the United States, however, people still pronounce the word as “erb”.