The first book in the New Testament is the Gospel of Matthew. It's author was a tax collector until he was called by Jesus to be one of His 12 disciples.
Matthew's Gospel was written about 20-30 years after Jesus had ascended to heaven. During that time the good news had spread throughout the Roman Empire, but Christians were starting to be persecuted. The aim of Matthew's Gospel was to strengthen the faith of Jesus' followers during these hard times, and also to give them a tool to help them spread their faith to others.
"Then the eleven disciples went away into Galilee, into a mountain where Jesus had appointed them. And when they saw him, they worshipped him: but some doubted. And Jesus came and spake unto them, saying, All power is given unto me in heaven and in earth. Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost: teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you alway, even unto the end of the world. Amen."
Jesus' command to His disciples is known as the Great Commission. It is through this that the first Christian Churches were founded, and the word of God was spread throughout the world