Actually i'm currently learning Korean online at talktomeinkorean.com Go take a look. It helps a lot. It organises the lessons into different levels, and each level have around 20 lessons:) Teaches you almost all the things you need to know and they joke around with each other too:) It is quite fun. There are audio and video lessons and there are also free pdf and mp3 file for you to download and learn. There used to be textbooks too, but are all sold out already(I think):) Have fun learning Korean. Hope this helps:)

Hi:) For casual, you use it to people who are younger than(younger siblings, friends who are younger than you) you or same age as you. As for formal, you use it to people who are older than you(friends who are a few years older) or have a higher status than you(teacher, parent etc). Hope it helps:)

This is not commonly used nowadays. It is more for when you met that person face to face and you are goin to leave. No harm using it, but if it is for phone, it is very uncommon. More for the olden days. People normally use bye bye(再见) or i'm hanging up(我先挂电话了) Hope this helps:) Have fun learning Chinese! Good luck:)