"I am in my house."
Translation:Ich bin in meinem Haus.
Jaye16, as I understand it, meinem is the dative/neuter form of the possessive pronoun "mein". It ends in "em" because it takes the dative neuter ending. It takes the dative/neuter ending because it follows the preposition "in". See: http://en.wikipedia.org/wiki/German_adjectives#Weak_and_strong_inflection
for some help with adjective endings, which, in this case, seem to match the possessive pronoun endings.