Shishinigami (soul reapers) are souls that were brought to the soul society by other soul reapers who perfomed a konso (soul funeral) or (passing on).
To become a soul reaper the soul must enter the soul reaper academy and get through eight years of training and then they will have the best chance of making it into the thirteen court guard companys (In mine there are about 20 to 25 companys.)
Once they enter the court guard companys they must work to gain the highest ranking possible wich is captain.The lowest ranking seat a soul reaper can have is tenth seat, anywhere lower than that they arent given a number.
All soul reapers from fifth seat up have to know the realease comand and name of their zanpakuto(shikai), but there is one exception and that is Kenpchi Zaraki, captain of 11th company, ultimate combat squad. Most of the captains know what most people think is the ultimate strength but for real there is a much stonger power than bankai.
Soul reaper kisuke urahara created a spiritual object called the Hogyoku (the break down sphere)that takes down a barrier betwein the world of hollows and the world of shishinigami, so then the two can become one and gain the powers of the other species.