As your characters collect orange Sparks, they earn experience and, when they have enough, they level up. Every time they level up, they also earn Skill Points that they can spend in the Skill Tree. You can get to the Skill Tree from the pause menu. Each character’s Skill Tree is different. It consists of skills that can affect health, speed, melee combat, ranged combat, the speed at which Power Discs recharge, and a unique Special Move. This move must be unlocked in the Skill Tree and can then be upgraded. For most characters, you want to get your special move as quickly as possible. Depending on your character, you may choose to focus on melee or ranged attacks.
The Skill Tree is called such because you can’t just pick any skill you want. You have to follow the branches of the tree to get to some skills, purchasing others along the way. Therefore, look at all the skills and find out which you really want and begin spending Skill Points to get to them. When looking at a Skill Tree, you may notice that some of the skills have laurels around them. These are skills that the game designers recommend and are very useful for that specific character. Try to include those in your acquired skills.