Saranghae is "I love you" and Oppa is what girls call boys if the guys are older than them. Basically, it means "I love you"
Buing buing is an action of earnest request. Koreans clench their two fists, put them on their cheeks and bob the fists up and down while saying "buing buing". If they do this, it is a sign that they are acting cute and most of the time, others would not be able to refuse their request. Basically, it is an action of acting cute to get what they want.
Waeyo means "Why?" in formal form. You use this when you are talking to someone you are not close to. In informal form, you should say "Wae?" it is just without the suffix but it still means the same.
Hope this helps.