John calls people out in the country to baptism, urging them to change their minds and look to God and His ability to forgive. By changing their minds they would become a 'new people' ready to receive the person of Jesus Christ.
Crowds joined him in the river and we get the idea that many were baptised. Yet all we know is that baptisms occurred. We don't know if John actually tipped people back, dunked them down or forwards, or even poured water over them as they responded to his call (neither do we know the age range).
We do know that they asked John how to live, and he replied with a neat summary of all our teaching from last year (!): share with others and live justly.
John's deal was to point towards Jesus. Yet when Jesus turned up he asked John to baptise him just like the others. Odd that - Jesus didn't need to change his mind, didn't need the Father's forgiveness for anything ... but he identified with all of us who do.
There was nothing magic in the water, nothing magic in John, but Jesus led us all by his own example. That is why churches nearly universally use water baptism as the mark of entering the global collection of people who desire to follow Christ (known more simply as 'The Church'). Virtually all denominations follow this logic, and therefore require that anyone wanting to formally be a member of a local church be baptised first. They may differ on how that baptism takes place (e.g. can young children be baptised, must it be full immersion and so on), but they agree that baptism is the outward sign that signifies the person joining the company of 'new people'.
Or to put it more simply: if it was good enough for Jesus, then it ought to be good enough for us!