Well, first, Й is a consonant. At best, it makes diphthongs, so "мой" rhymes with "boy"
И is a vowel, and it is stressed in "мои". Just like "foresee" and "force", which are quite different, мой and мои are not that hard to tell apart.
The form of мой follows the case of the noun it is attached to and also reflects ts gender and number.
As usual for adjectival declension, neuter forms are pretty much the same as masculine ones, and there are very few different feminine endings.