The currency the game uses is coins and there are several ways you can earn them. You will be awarded coins as you progress through the game and the amount you receive is determined by the distance you cover running and killing zombies with the weapons you have at your disposal. Upgrading your weapons (including secondary weapons) will allow you to earn more coins from zombie kills as will destroying gas stations and rescuing survivors. Basically the more chaos you cause the more coins you will be awarded. Also, don't forget to tap on the coin icons when they randomly appear on screen as this will get you even more coins. Spend the coins you earn on leveling up your character by purchasing skins, weapons, and boosts.