When Shinigami die, they return back to Earth and reincarnate here. When they die here, they go back to the soul society.
This cycle continues till the souls become tired of this cycle, and they get lost in the valley of screams, where their memories are removed from their soul bodies. I'm not sure, but I think from this new souls are created. This was explained in the movie Bleach: Memories of Nobody.
As to how souls get to the soul world, my impression from season 2 is, that they just make their way their naturally using one of the many walkways, and randomly land in one of the districts. The process seems haphazard, as people land anywhere, & then spend their time trying to find their relatives, which they rarely do.
As for hell, I think the Manga author muddled it a little. In season 1, hell is reserved for very evil souls, like the serial killer. In the bleach movie part 4, the main villain had only killed those who killed his sister, but is still sent to hell. This seems like a plot hole, as many people in Soul society, like Aizen, are a lot more evil than anyone in hell.