It depends on the gender of the noun (which you must check in the dictionary when You're not sure) and on the singular/plural forms, e.g.:
mój pies - my dog (singular male)
moja książka - my book (singular female)
moje dziecko - my child (singular neutral)
moi rodzice - my parents (plural male person only) - because 'rodzic' (parent) is male gender in Polish
moje psy, książki - my dogs, books (all other plural forms) - note that 'dog' is male, but it is not a person
Note that 'moje' goes with both (singular and plural)