You will automatically get your first assassin as you play the game trying to liberate the various parts of the cities. More will join you as you liberate the various districts. To level up an assassin, hold [LB] and press [X] on any of the available actions (for contracts). Choose the various contracts and send off your assassin on the missions that seem to be easy enough to complete. If failed, you will have to wait for the assassin to recover. Get enough experience points to gain the final rank (1100XP). There are a total of 84 missions.