The way you rank up your assassins is through gaining experience mostly through something called Mediterranean Defense. Any assassin den or the assassin boards next to the bomb crafting tables in the cities have the access point for managing assassins. It is also in the Assassin HQ. There are 13 different city names, each with various tasks. This will be explained in more detail in The Mentor achievement description. The main task you will need to complete in each is "Reclaim The City". This means it is under Templar control and you must send assassins to take it over. Pick the assassins that give a high probability and send them. It takes 10 minutes for them to complete the mission. You can get this achievement done as you level them up at your leisure, but keep in mind you gain more money by having control of the cities.