You can say an informal goodbye by saying 안녕! That doubles as an informal hello. There are two ways to formally say goodbye (to my knowledge anyway) but they're pretty long. 안녕히 게세요 if you're leaving someone's place, or 안녕히 가세요 for someone leaving your place.
Wait - 기다려 (informal), 기다려 주세요 (formal, there are lots of ways to say this)
Who are you (or who is it)?- 누구야? or 누구니? (informal), 누구세요? (formal)
You've got the formal way down to say thank you. Here's the informal: 고마워 or 고맙다
What? - 뭐? or 뭐야?! (this one mean's more "what is it?" but can double as a surprised WHAT?!, both are informal) 뭐에요? for the formal I think, I'll have to get back to you on that one, it's been way too long since I've had a good convo in Korean.
I also think the "ㅂ" sounds like the American "b," but my boyfriend (who isn't Korean) says it sounds like a soft "p" sometimes, especially when spoken quickly (but I still say it sounds like a b :P) I might have some spelling errors >.< (I'm great at speaking and reading, but writing or typing is pretty difficult) so I'm sorry if I do, I'll check on that with my mom and sister later to see if they are error free. But as far as saying them they should sound right. Hope this helps!