And having slain the king of the Rakshasas, with his brother, and sons and kindred, he installed in the kingdom in Lanka the Rakshasa chief, Vibhishana, pious, and reverent, and kind to devoted dependants. Then Rama recovered his wife even like the lost Vaidic revelation. Then Raghu’s son, Rama, with his devoted wife, returned to his own city, Ayodhya, inaccessible to enemies; and that lord of men began to dwell there. Then that foremost of kings, Rama was established in the kingdom.

In plain words: After killing Ravana and his family, Rama made Ravana's good and kind brother Vibhishana the king of Lanka. Rama got his wife Sita back, went home to his city Ayodhya, and ruled as king.