Fortunately, you do not have to be a lawyer to play Phoenix Wright: Ace Attorney Spirit of Justice. The game makes up its own rules regarding the justice system. Your customers are not necessarily guilty, a case can not last more than three days. Nevertheless, if you make too many mistakes, the judge will declare your client guilty and it will be a "Game Over", so it's advised that you to save the game as often as possible.
It's also advised that you to read the section Phoenix Wright: Ace Attorney - Spirit of Justice/Characters.