Despite the gruesome agony of the cross (see How Did Jesus Christ Die?), one of Jesus' last words was for the care of His mother Mary, who at that point must have been in a state of extreme grief and horror upon witnessing The Crucifixion. From then on, she was to reside as family with John, one of the closest friends of Jesus, and apparently one of the bravest - he remained there with Jesus from the "trial" right to the end, despite very real danger to himself, while many others, including Peter, had run away (see The Fateful Night).
John and his brother James were the third and fourth fishermen (see The Fisher) of the Sea Of Galilee that Jesus had called to service that day (brothers Andrew and Peter were the first two). John and James had been fishing with their father Zebedee and left immediately to follow Jesus. Zebedee was apparently relatively wealthy, and had other hired men there, so John didn't leave his father all alone (Mark 1:19-20). They had probably heard the preaching of John The Baptist, who had just been imprisoned (Mark 1:14), and were aware that a Savior was coming, although they may not have realized that they themselves would be called to service. Either way, they were aware and prepared - they immediately followed Jesus.
John eventually became a very close disciple, and friend, of Jesus. Jesus' nickname for John and his brother was "Boanerges" which means something like "Sons of Thunder" - evidence of John's zeal and temper, and perhaps of Jesus' sense of humor.
After the resurrection, John and Peter were the first people to be told by Mary of Magdala, who had discovered the empty tomb (see The Garden Tomb) early that morning. The Bible states that, at that point they did not yet fully realize or understand that Jesus had actually risen - they thought that someone had stolen the body (John 20:1-9). John and Peter quickly ran to the tomb (John was apparently more physically fit than Peter, since he arrived at the tomb well ahead of Peter) and discovered the empty burial cloth and strips of linen.
John and Peter then left for home, but Mary of Magdala remained - and certainly to her credit, because two angels then appeared to her along with the risen Jesus Christ Himself, but she did not immediately recognize Him (John 20:10-18). Mary of Magdala was among the first three people (Salome and Mary the mother of James were the other two) who together discovered the empty Tomb, and she alone was the first to see the risen Jesus Christ.
Despite the persecution that he faced through the many years of his service and witness (that resulted in the martyrdom of nearly all, if not every one of the other apostles - see What Happened To The Apostles?), John managed to live to old age because (apart from caring for Mary who was likely not more than age 50 at the time of The Crucifixion) he had yet more writing to do. While banished to the island of Patmos, after having already written the Gospel of John (of which Mary would have been a very good source of information), he was given to write the Book of Revelation.
Fact Finder: What is the origin of the Book of Revelation?
(a) from John to us
(b) from Jesus Christ to John to us
(c) from God to Jesus Christ to John to us