You may be aware that, according to Matthew, John the Baptist himself raised the precise question you have: Why did Jesus need to be baptized? The short answer is that He didn't, but that He chose to do so as He chose to many thing to provide a righteous model for His subsequent followers. Baptism initiates the commitment to Christianity that many denominations complete with confirmation. That it IS a commitment is why many Christian denominations (most famously the Baptists) don't condone infant baptism; IMHO, it suffers the dual liabilities of being both presumptuous and pointless, no more meaningful than taking a national oath of allegiance on behalf of ones infant child. Baptism's predicated on repentance of sin and acceptance of Christ as Lord and Savior from it, but it's not like those who do those things yet never have the opportunity for baptism are denied salvation. Christ told one of the thieves crucified beside Him he would be with Him in paradise, and I'm pretty sure no one baptized him in the interim.

As far as Johns authority to baptize, it comes in two parts: First, ritual purification by washing is part of Jewish tradition at least as far back as Moses (IIRC washing before every meal is part of the Torah for reasons more obvious now than they were then). Additionally, Johns particular commission to baptize was in anticipation and on behalf of the Messiah Whose coming he preached; in essence, he baptized people for both their present repentance and future faith in a Savior not yet revealed, but imminent.

EDIT: Y'know, displaying this thread in a paged format could have saved me the trouble of making this post (which reiterates points various knowledgeable people had already made) and let me go straight to the more narrowly focused ones I made elsewhere (without the need to make three posts instead of one). Not a criticism, just an observation.