The Sixth Faction by Veronica Roth

Beatrice Prior stands on the brink of a decision every person in her world must make: five factions, five futures, five lives. She has a plan: she's going to make the bravest choice, and leave Abnegation behind to join Dauntless, even though it means leaving her family behind. But when tragedy strikes at her Choosing Ceremony, everything changes—her world, her plans, even Beatrice herself. Her ensuing decision thrusts her into the crosshairs of an underground rebellion, and into the path of a boy with secrets as dangerous as her own - a boy with only a number for a name.

One choice will change everything.