Possessive adjectives agree with the noun they describe, not with the owner's gender — a common trap for English speakers. Mon is used before a masculine singular noun (mon frère, my brother), ma before a feminine singular noun (ma sœur, my sister), and mes before any plural noun regardless of gender (mes parents, my parents).
Before a feminine singular noun that starts with a vowel sound, ma is replaced by mon purely for pronunciation: mon amie (my friend, a woman), not ma amie. The same pattern extends to ton/ta/tes (your, informal) and son/sa/ses (his/her/its) — son livre can mean either "his book" or "her book", since the possessive agrees with livre, not with the owner.