Give your hero a birth, metaphorical or literal, establishing him as a hero in training. Harry Potter defeated the most evil wizard ever as a baby; the lightning scar he carries brands him with his own mythology.
Sound the call to the quest. For Harry Potter, the journey starts when he begins attending Hogwarts, a rite of passage that sends him into a new world and sets into motion a new chain of events.
Give your hero some helper characters. These can be friends, mentors, or both. Harry Potter, of course, has the wise old Dumbledore, plus his two best friends, along for the ride.
Make your hero refuse the call to his destiny. All heroes, tween or otherwise, experience doubt at some point. One of Harry Potter's came during "The Order of the Phoenix," when he didn't want to be the "chosen one" anymore.
Establish a nemesis character for your tween hero. This character will be whatever you make it, human or otherwise. Harry Potter's is Voldemort.
Test your tween hero with trials, each trial being harder than the last, in the lead-up to the climax. In the Harry Potter series, of course, there are multiple trials for Harry to face, in the ultimate lead-up to the final book.
Write a big battle for the tween hero that will climax the adventure (at least, for now). Each Harry Potter book has its own big battle moment; the overarching climax of the series is when he defeats Voldemort.
Return your tween hero to his regular world. He should be changed in some way for the better, having attained knowledge on his quest. Harry Potter must always return to his beastly relatives, but secure in his new magic knowledge.