In English spelling, Y can be a vowel or a semivowel (you can look up semivowel online — the wikipedia will tell you more than you need to know), depending on the word.
Here are some very quick thoughts:
Y as a vowel: myth, study
Y as a consonant (actually a semivowel / semi-consonant): you, beyond (also, the U can begin in this sound: use)
Y as an off glide (a semivowel, but usually analyzed as a vowel in English and Portuguese): pay
Y as a vowel followed by a glide: scythe, analyze
Covering the subject would take a book at least, but if you tell me why you want to know (are you concerned with spelling, pronunciation, trivia?) I’ll be happy to fill you in.