The turning of water into wine must be read alongside the second story in chapter 2, where Jesus cleared the temple courts of the non-religious nonsense that was going on. There is a reason why John places this incident at the beginning of his gospel rather than towards the end. The other three gospels place this event in its historical location, during the last week of Jesus' life, in Jerusalem. John, while writing about historical things, writes much more of a symbolic gospel. So John wants this event to be understood under the symbolism of the water that was turned into wine, which is why he places it here.
What have we covered thus far in John's gospel? We have seen the "prologue," where Jesus is described as the incarnate, revelatory "word" of God. We have seen John testify about Jesus. And then we have seen Jesus call disciples. It is after the calling of disciples that Jesus begins his ministry. And what is the first thing he does? He turns water into wine. There is a double signification here. The first signifier is seen in the comments of the banquet master: You have saved the best wine until the end (10). Thus, Jesus is better than what has come before; he is an improvement upon it. He is the fulfillment of what has come before, and this is signified through the transformation of water into wine.
This is further signified in combination with the cleansing of the temple. Jesus is the purifier of the Jewish religion. Fermenting grape juice into wine was one way in which Jewish people were able to drink water that had been, in a sense, purified. Water was not always clean. In many cases it was impure. Wine, through the fermenting process, was an agent of purification in that it enabled them to drink without becoming ill. Likewise, Jesus went into the temple and purified it, indicating that worship should be characterized by zeal for God, not by money. He also spoke about the temple metaphorically, that the temple (his body) would be broken down but raised up again. It is through his body, broken for us, that we receive the purification signified in John 2.