I'm reading the New Testament and have noticed Matthew and Mark are almost the same. Why is this?
The similarities of the books of Matthew and Mark, probably has more to do with the time frame in which they were written and the people to whom the books were written; than any collusion by the authors.
The books of both Matthew and Mark were written during the early church period and were probably mostly complete before the Apostles went separate ways. These books were written to basically the same audience, and during that time frame there was most likely a rehashing of the events over the three years of Jesus ministry. The fact that these were basically uneducated Galileans, almost ensures that they would have devised some means of remembering those things discussed and may even be the beginnings of icons in the Orthodox Church.
If we take a look at Acts we can ascertain that there was a period of several years before the core of the church began to disperse since most of them were expecting Jesus to return during their lifetime.
Of the three synoptic only Matthew and Mark were written by Apostles who were present to witness the events, and therefore their accounts coincide; it would be strange if their accounts differed greatly. These two Gospels were written to people who would be familiar with Jewish history, however Luke's Gospel was written to Gentiles, and therefore had to be more inclusive than those. That is apparently why Luke begins with the conception and ministry of John the Baptist.
The Gospel of John the Apostle was written as a witness to Jesus Deity, while the Synoptic Gospels were written to show the humanity of Jesus.
Hope this helps.