Percy Jackson and the Olympians by Rick Riordan
He does this with the help of his friends, and the fact that he is the son of one the Greek gods, all of whom (along with the monsters) have survived the past two millennia and migrated to America.
The first book of five, The Lightning Thief, begins with Percy having a weird experience on a class field trip, during which his Pre-Algebra teacher turns into a evil bat lady and he vaporizes her with a sword that expands out of a pen. Most kids would have just decided to stop buying stuff from that bearded man on the corner, but when he arrives home and takes a trip to the beach with his mother and his best friend Grover, both of them seem agitated and tense. Then, one stormy night, his mother drives through the Long Island countryside to a place called Half-Blood Hill, even as they're chased by the big daddy of all bulls, the Minotaur. The Minotaur grabs Percy's mother and she vanishes into a shower of light. Percy uses his Awesome Demigod Powers to kill the Minotaur, and then wakes up later to discover he is in Camp Half-Blood, a training camp for children of the gods. The rest is history, but the kind you probably don't know yet, so I won't tell you.
The other four books take place in the following summers (except for Book 3, which occurs during the winter) as Percy grows up, kills monsters, and does all-around awesome things. Along the way, he meets a very well-developed and dynamic cast of characters, like Annabeth Chase, the intelligent and resourceful daughter of Athena, Nico di Angelo, the dark and brooding son of Hades, and Rachel Elizabeth Dare, a mortal girl with surprising powers.
The prose is fast-paced and generally good, but this area is where a lot of the problems (read: only about two in all) that I have with the series lie. First, Percy breaks the fourth wall a lot. Sometimes it's nice, but other times it can be awkward. Riordan also has a habit of dumping laundry-list descriptions of characters at certain points in the stories. But all in all, it isn't too big of a deal.
My favorite thing about the series is how well Riordan assimilated the gods and monsters into 21st-century America. Apollo, god of the sun, music, and poetry drives a bright red Maserati (which actually is the sun) and makes up haiku and limericks on the fly (even though most are awful). Hephaestus, god of the forge, fixes cars in his spare time. The monsters are very well-hidden, and if you can interpret the signs before they're revealed and guess what monster it is, then you are awesome. So I won't spoil any of them. Riordan does take a few liberties, but I do not hold it against him by any means.
There are also some just plain hilarious moments. In the fourth book, Percy witnesses a classroom filled with young monsters watch a video about puberty and growing up (although then proceeds to kill a large number of them). In the third, Percy and some other characters visit the Hoover Dam and talk about finding the "dam snack bar" and getting some "dam French Fries", much to the confusion of one character, who still uses "thee" and "thy" and has no idea what's so funny.
This series, while no masterpiece, is one of the best-written pieces of fiction I have ever read. I recommend this to anyone who likes fantasy, Greek mythology, or breathing.