When the multitudes were seeking Jesus, one of the questions they asked is what to do to "work the works of God" (6:28). But before talking about what they should do to do the "works of God", Jesus discussed what is "work of God".
Jesus responded that the "work of God" is to "believe in Him who He sent" (6:29). We know from John 3:16 and John 5 that God sent Jesus to Earth. Thus, believing in Jesus himself is officially considered a work of God, which is further supported when Jesus reveals himself for us to believe in him. As stated in previous verses like 3:16 and 5:24, believing in Jesus nets us eternal life and crossing from death to life.
So everyone who has believed in Jesus has done the work of God! If you're curious on believing in Jesus, I highly encourage you to read John 3:16, John 5:24, and my corresponding articles.