To get either of these endings will depend if you decide to kill Ryoji or not. To get the good ending spare Ryoji's life on the 31st of December and for the bad ending choose to kill him.

When you have successfully completed the game 'Save' it and then when you load a new game with the clear game data you will start the game with your previous level, equipment, gameplay time, your demon compendium, maxed Social Links and access to the Monad Block. You will also be able to fight an optional boss and any consumable items in your inventory will disappear.

This is done if you manage to reach Tartarus at the 254th floor which isn't easy as the monsters in Monad are over Level 90+ (there is an escape point on the 10th floor but no checkpont).

After you have completed the game, you can restart with your current levels, money and other stuff. To do this, just save your completed game, then load it up again and you should be able to start with all your gear.