the best line is:hey what's up
that doesn't really open up conversation... they will just say 'hey' or 'nothing'.
a good line is 'you look like a nurse' (or teacher, librarian, etc), then they will correct you, then you say something about said occupation
girls are usually with other girl friends, so it's good to talk to all of them first and say something like 'hey you look like close friends' then they will be compelled to answer.