There is a long and short of the question and of the answer.
The EASIEST account: Salvation occurs when a person accepts the fact that Jesus Christ was the Son of God, and that he died on the cross in order to win forgiveness for the sins of all mankind.
"For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life. For God sent not his Son into the world to condemn the world; but that the world through him might be saved."
However, if you read on, you'll see that Jesus instructed his disciples to do considerably more than believe in his sacrifice.