"нету" is a variant of "нет" in the sense of "absent". It can be replaced by "нет".
"не" is a negative particle.
"ни" is an intensive particle in negative phrases. Mostly equivalent of "any".
"нету ни рубля" if translated word-by-word is: "no any rouble". It requires Genitive.
"нет приличных людей" - no decent people (it is always Genitive after "нет" in the sense of "absent")
"среди моих друзей" - among my friends ("среди" requires Genitive)