"I am not at home."
Translation:Я не дома.
Well, it's pretty straightforward. «Дом» means house, home; «дома» is a different, but related word, which means "at home".
Я не дома — I'm not at home.
Я не в доме — I'm not in the house.
I must add that the latter is a bit odd, but possible in situations like «Я в Доме культуры.»
Because the accent is on the last syllable. Therefore the O is unstressed and gets pronounced as A