The beginning of the Gospel of Jesus Christ (v. 1)

Gospel, in it's simplest form, means good news. For Mark, the good news is that Jesus, the savior prophesied of throughout the Old Testament, has arrived.

Isaiah 40:3 (vv.2-3)

John the Baptist (vv. 4-7)
John's role, according to the prophecy in Isaiah, was to prepare the way for Jesus.
John's message – Repentance for the forgiveness of sins and to be baptized as they confessed.

Baptism of Jesus (vv. 9-11)
“Mark leaves no doubt for his readers as to who Jesus is: God’s voice declares that Jesus is his messianic Son.”
- Schnabel, E. J. Mark: An Introduction and Commentary

Temptation of Jesus (vv. 12-13)

“Jesus is not Zeus. He was a sinless man, not a sinless Superman. He woke up with bed head. He had pimples at thirteen. He never would have appeared on the cover of Men’s Health (He had “no beauty that we should desire Him” - Isa.53:2). He came as a normal man to normal men. He knows what it is to be thirsty, hungry, despised, rejected, scorned, shamed, embarrassed, abandoned, misunderstood, falsely accused, suffocated, tortured, and killed. He knows what it is to be lonely. His friends abandoned Him when He needed them most; had He lived today, every last Twitter follower and Facebook friend would have un-friended Him when He turned thirty-three — He who will never un-friend us.”
Dane Ortlund
Gentle and Lowly: The Heart of Christ for Sinners and Sufferers

Repent and Believe (vv. 14-15)
Turn from your sin and give your life to Jesus, trusting in the only One who can save you.

Follow Me (vv.16-20)

“Both pairs of brothers found that obedience to the call of Jesus was costly; it meant abandonment of all that they held dear, and all earthly security, in simple committal to Jesus... Both left all that they had; that is always the minimum requirement for the Christian.”

Alan Cole
Mark: An Introduction and Commentary