This requires you to spend money to upgrade each character's abilities to the 5th and final tier. Each ability must be used and leveled up prior to being able to be purchased, allowing it to be further leveled up through use.
Speed - You fill up the bar as you run around with your character
Default Weapon - The default weapon is the character's attack. Just keep on shooting to fill up the bar.
Zombie Bait - Each character has a unique Zombie Bait which can be used by pressing as long as they have "ammo" for it. Use these whenever you can to fill up the bar.
Pwnage Ability - Once the tube next to your character's picture fills up, press to use their Pwnage ability, and fill up the experience bar.
Melee Weapon - Tap to melee and level up this skill.