There are 171 skills in the game and unlocking all of them is quite a challenge and time consuming. You can look in the Skills Menu to see what the requirements are for unlocking each skill. Sometimes they are simple and only require you to complete a mission or do so using a specific skill. Other times they are more difficult and require you to achieve a high score or rank on a mission. Unlocked skills appear brighter in the Skills Menu, while locked skills appear dim. Skills with a question mark are only obtainable on a mission that you have not available yet. Complete more levels in Mission Mode to unlock more missions to make all the skills visible.