While the achievements in this game are fairly simple, you will spent the longest time doing all the quests and instincts you see on the map.
Before you reveal the entire map, you should have found all of the Goat Towers (Assassin's Creed anyone?) which are both the main hub of the game and the way to teleport over the map to cover big distances. Each part of the map will have question marks showing all sorts of quests. Each quest give you a certain amount of points called Illuminati Points which ranges from 1 point to 5 points.
S Rank is the final rank which requires 175 points to unchain the door to the final area of the game. You can look in the menu and see how much progress you made.