The game is about the story when Ezio is born and the main enemy of the Templars are Giovanni Auditore da Firenze. And the Templar is a guard during the Renaissance.
The Templar Guard starts on the militia rank. To rank up he would need to kill some assassins in order to rank up. The next rank is Elite and Archer. There're some special ranks like the archer or brute for other choices if you don't want to original one.