Aminah can refer to two different Arabic names. The first one is Ameena, the second one is Amina. In the first one, the emphasis is on the second syllable, while in the second name the emphasis is on the first syllable.
You can use Aminah to refer to either of the names. How you pronounce it determines which name it is. If you are looking for the name of Prophet Muhammad's mother, peace and blessings of Allah upon him and his family, her name is Amina (the second link above), pronounced like aam-nuh.