Jesus gave the task of telling the story to his followers. Jesus did not write a book like many other famous people nowadays write their autobiography. Writing the gospels was part of the missionary task. So also the letters of Paul and other letters in the New Testament were just one way of telling the good news. Christians had the practice of reading gospel writings and missionary letters when they came together. Speeches were given based on these, as also based on the Old Testament scriptures. The Holy Spirit led the church to see the Bible as a whole, where there is both the Old and New Testaments (2 Pt 3:15-16 & Rev 22:18-21).
The collection of books found in the Bible is called canon. Jewish rabbis held a meeting sometime around the year 100 AD, where they finally defined the Old Testament collection of books. The Christian bishops had a similar kind of meeting in the year 397 AD, where they defined the canon of the New Testament. The practise of the local churches had however already set the borderlines for the New Testament, as books were being copied and distributed to different towns of the Roman Empire. Some books, however, were traveling apparently slower than others. Therefore there was some discussion, if for instance the Letter to Hebrews, Letter of James and Book of Revelation really belong to the Bible. Not all churches had these books. The Holy Spirit had led the church together to recognise the holy writings of the New Testament as it is. The Bishops’ meeting just sealed the fact.
Handwritten New Testament manuscripts from the first centuries have been found in their thousands. Scholars study these and the minor variations and mistakes which may be detected.The earliest fragments of the New Testament are from the first century. One of the oldest whole New Testaments is the Codex Sinaiticus from the beginning of the fourth century. You may visit the British Museum in London to see this collection yourself.
Open the Gospel of Mark and read some of it. You will notice that it is written in a different way than, for instance, this writing here. Mark is written according to events. Alongside the events we read about the teachings of Jesus. This means that the whole life of Jesus was the word of God, not only his teachings. Jesus is the Word of God (John 1:1-14). Disciples are the eye-witnesses of the Word and write their testimony down in the Gospels. The letters of the New Testament give the teaching which was needed in the first congregations. If you study the Letter to Romans, you will notice the long doctrinal part. First Paul indicates how everyone is sinful and lost. Then he teaches about salvation as a gift of God to those who believe in Jesus. Thirdly he teaches how to live as a Christian to the honor of God. We may follow the New Testament models in our testimony about Jesus.