The Last Kids on Earth by Max Brallier

It's been forty-two days since the monster apocalypse hit town and pretty much everyone ran away or got zombified. Average thirteen year old Jack Sullivan is holed up in his tree house, which he's armed to the teeth with catapults and a moat, not to mention video games and an endless supply of Oreos and Mountain Dew scavenged from abandoned stores. But Jack alone is no match for the hordes of Dozers and Winged Wretches and Zombies, and especially not for the eerily intelligent gargantuan menace known only as Blarg. Jack needs to build a team of whatever kids he can find, and whichever monsters will show him loyalty. With their help, Jack is going to slay Blarg, prove himself the ultimate post-apocalyptic action hero, and be average no longer! Can he do it?