For most activities that you perform in the game, you receive experience points and get promoted to higher levels. Whenever you gain the next experience level, you receive a point that can be spent on a special skill. The skills are divided into three paths: Heron, Shark, Spider. Heron skill focuses on eliminating enemies over large distances, aerial attacks and prolonging the effectiveness of the syringes you inject yourself with. The Spider path improves bow skills faster switching/reloading of weapons and efficient movement. Among the Shark path, there are talents that increase your health bar by further segments, chained takedown and a very useful Heavy Beatdown.
As far as the number of points you get, is concerned, the game prefers those who prefer silent elimination. A good example is killing an armored enemy with the Heavy Beatdown skill. For an enemy killed this way, you receive 500 points. That is why, you should first turn your attention, and develop, skills of silent assassinating. Then you should decide on the skills that mach the type of weapon that is most frequently used.
You can easily buy the skills from the path of your choice if you perform side missions. You gain access to the remaining ones alongside with your progress in the game. Some of the skills have additional requirements. E.g. in order to unlock Adrenaline Surge, you first need to perform three supply drop missions.