"Bye-bye"
You can say in many ways.
With the elder people or your teacher you should say the same word with 'Hello'. You'll say
สวัสดีค่ะ/ครับ (Sa-wad-dee-ka/krup)
But with the younger people or your friends you should say
บ๊ายบาย (Bye-bye)
It's like English, huh? But the first syllable you'll say a little higher pitch.
Just try :)