In Arabic, you show possession by using endings that get added to the end of the words, unlike in English. To say that something is "mine", you have to add ـي to the end of the noun. There are two simple rules you have to know:

•Nouns ending in a consonant (mostly masculine): i.e. beit (house) ---> beiti (my mouse) بيت ----» بيتي

•Nouns ending in ta marbuta (mostly feminine). becomes a before the ending gets added to the noun. i.e. madarasa (school) ---> madarasati (my school). مدرسة ----» مدرستي

I hope my explanation was helpful. Good luck in learning Arabic!