안넝하세요..is means hello, a formal greeting, it is all the same meaning by english good morning, good afternoon, and good evening.
안넝.. is informal for friend family that close to you..
but in telephone don't say 안넝하세요 when u answer the call its another meaning it is 여보세요.
안녕히 가세요 mean good bye.. it is formal way,older than you or people have in higher position.
안녕 for close to you it is informal word..
i also beginner in korean so i guess i can answer your question