The bulk of what we have today attesting to the existence of John the Baptist can be found in the New Testament, however the historian Josephus mentions him in the 18th book of his Antiquities of the Jews (chapter 5), and most modern historians agree that he certainly existed since he is also attested to outside of religious literature. He was a Jewish itinerant preacher who apparently was a member of the apocalyptic sect of the Essenes. According to Josephus, he taught repentance of sin and encouraged people to be immersed in the river Jordan for reasons of ritual purity.
The New Testament has some rather interesting input on his ministry, but before discussing that, I should explain some of what Christian scholars say about the gospels so that we can get a picture of what the New Testament does with John. The four gospels were written in a different order than the order they are found in the New Testament, all scholars agree on the order and approximate dates of composition. Mark was written first about 70-80 CE, followed by Matthew and Luke being written about 15 years later (85-95 CE), followed finally by the gospel of John about 10-15 years later than Matthew and Luke (95-110 CE). The entirety of the gospel of Mark is contained in Matthew and Luke with some minor variation in the content, and all the New Testament scholars agree that when the gospels of Matthew and Luke were written, the authors had Mark as a source and they also had a source that's referred to simply as Q from which a number of teachings of Yeshua/Jesus that were included in Matthew and Luke (but not in Mark) were gleaned. Q is believed to date to the 40s, but might actually date back to the lifetime of Yeshua/Jesus, and it's thought that Q is the best and most original source of the original teachings of Yeshua/Jesus. So, at least some of Q is included in the gospels of Matthew and Luke, and it represents a tradition of Yeshua/Jesus' teachings that predates the rest of the New Testament, including the letters of Paul (which were written in the 50s).
What does the New Testament have to say about John the Baptist? The gospel of Luke quotes Q in Luke 11:1-4 and shows one of Yeshua/Jesus' disciples asking him to teach the disciples to pray a specific prayer that apparently Yeshua/Jesus learned from John the Baptist. According to an ancient Christian oral tradition, John the Baptist was the rabbi of Yeshua/Jesus. Again quoting Q, Luke 7:24-30 shows Yeshua/Jesus giving John the Baptist the highest praise, even going as far to say that there was never a prophet greater than him. In Q, Yeshua/Jesus was apparently a disciple of John the Baptist and has the utmost respect for him, and he heavily praises and magnifies John the Baptist in front of his own disciples. In Q, John the Baptist has a very lofty role and is held in the highest esteem even though he wasn't a follower of Yeshua/Jesus.
In the gospel of Mark, John the Baptist's role and esteem is downplayed a bit as Yeshua/Jesus rises in esteem. Mark 1:9 mentions that Yeshua/Jesus went to be baptised by John the Baptist, but neglects to mention the high praise from Yeshua/Jesus that Q mentions. So John the Baptist still has a great role in Mark, but not as great as in Q. In Mark 1, we can see that it's no longer John the Baptist teaching repentance to bring about the kingdom of G-d after Yeshua/Jesus' baptism like it was in Q, but now we see that Mark has given this role and message to Yeshua/Jesus. So we can see that John the Baptist has a lower esteem in Mark than he did in Q, while the esteem of Yeshua/Jesus is higher.
In Matthew we see that John the Baptist again loses some esteem as it's given to Yeshua/Jesus. In Matthew 3:11-17 we can see that unlike in Q, where Yeshua/Jesus has nothing but praise for John the Baptist and says that there has never been a greater prophet than John the Baptist, now John the Baptist has nothing but praise for Yeshua/Jesus and even goes so far as to be reluctant to baptise Yeshua/Jesus and instead says that he needs to be baptised by Yeshua/Jesus. This is only found in Matthew, so it's clear that it was an innovation of the author of Matthew and he wasn't quoting from another source. Again, we see the trend of John the Baptist's role and esteem being taken away and given to Yeshua/Jesus.
In Luke 3:21, we read that Yeshua/Jesus was baptised, but John the Baptist is completely taken out of the story. In Q, Mark, and Matthew, Yeshua/Jesus is clearly baptised by John the Baptist, but Luke removes John the Baptist from the story completely as if he has no importance. In fact, in the verses just before Yeshua/Jesus' baptism, John the Baptist is taken to prison, so it appears as if John the Baptist isn't even there to baptise Yeshua/Jesus. This is only mentioned in Luke, so it's clear that it's an innovation of the author of Luke, and he wasn't quoting another source. We can see that there's a growing trend in the gospels now where John the Baptist's role is gradually downplayed and all the esteem is given to Yeshua/Jesus as his role gradually becomes greater to the point of John the Baptist almost completely disappearing from the story as the time between the life of Yeshua/Jesus and the composition of the gospel increases.
In the gospel of John (not written by John the Baptist), the baptism of Yeshua/Jesus is implied, but never mentioned. Instead, we see that John the Baptist has the highest praise for Yeshua/Jesus and mentions that he was talking about Yeshua/Jesus when he said that there was coming one after him who would baptise with the Holy Spirit, something never mentioned in the earlier sources.
So we can see that the further from the lifetime of Yeshua/Jesus the gospels are written, the role and esteem of John the Baptist is gradually taken away and given to Yeshua/Jesus. This is the type of revisionism that we would expect from a work of fiction that might be based on historical events as time progresses and the eyewitnesses of these events die off and can't be consulted anymore, but we could never expect this from the eternal word of G-d. What's most disturbing about this is that the story of Yeshua/Jesus is very obviously being gradually exaggerated over time, and it happens in the Christian Bible for all of the Christian world to see, but just a few Christians have spoken up about it. The reason that so few Christians have brought this up is probably because most Christians read their Bibles either by small passages at a time without cross checking the parallel versions in other gospels or by reading the gospels in order and they've forgotten the details of what they read before by the time they reach the parallel versions. In fact, churches that have a liturgical cycle generally read the gospels in the order that they are printed in the New Testament, and the congregants generally forget the details from gospels they've already read as they reach the next gospel. If they read the parallel stories in each of the gospels at the same time, they would notice this trend of exaggeration outright, but this isn't how most Christians read the gospels.
So what does the New Testament do with John the Baptist? The authors of the gospels habitually take away from the role and esteem of John the Baptist and give that role and esteem to Yeshua/Jesus. Since the New Testament is so willing to rewrite and revise the story of John the Baptist and Yeshua/Jesus as time goes by, can it really be trusted to contain the unaltered word of G-d? Can it really be trusted to give the truth about Yeshua/Jesus? Can it be trusted at all? Psalm 19:8 (verse 7 in Christian Bibles) says "The Torah of HaShem is perfect, restoring the soul. The testimony of HaShem is trustworthy, making the simple man wise". If the New Testament is really the testimony of G-d, it would be trustworthy and make uneducated people wise. But since the gospels exaggerate and revise things, and this can be seen by reading parallel passages in the gospels, they obviously cannot be trustworthy. At the very least, they cannot be trusted enough to accurately teach the word of G-d. If your father told you a story that might have been based on actual events that you can't verify, and he changed the story every time he told it, would you trust him to tell you the true story?