I thought Brahma was the creator. Does Brahma create this world, or does Krishna?

Krishna Himself doesn't have to "do" anything; He's the supreme enjoyer, the emblem of eternity, knowledge, and bliss. He arranges for others to work according to their qualifications and desire to serve Him. He's always engaged in loving pastimes—lilas—with His devotees, while Brahma is the person who actively carries out Krishna's desire to create the material universe.

First, Krishna simply desires that there be creation. He expands Himself into Vishnu forms, then Brahma appears from Vishnu (see Srimad-Bhagavatam, 3.8). Brahma, as a faithful servant of Krishna, then prays to Krishna for inspiration and creative energy before beginning his task.