When you have clocked the game, save it when you are given the option of unlocking "New Game +" mode.

Basically this option lets you start off another game however from the start you will have all of your weapons, armor and souls from the time you clocked the game (keys etc are not included).

Note also that the health and power of most enemies will also be increased during this second time play through. This will make it much more difficult during the later stages.