The story begins on the night of July 18-19, 1830. A child (perhaps her guardian angel) awakened Sister (now Saint) Catherine Labouré, a novice in the community of the Daughters of Charity in Paris, and summoned her to the chapel. There she met with the Virgin Mary and spoke with her for several hours. During the conversation Mary said to her, “My child, I am going to give you a mission.”
The Second Apparition
Mary gave her this mission in a vision during evening meditation on November 27, 1830. She saw Mary standing on what seemed to be half a globe and holding a golden globe in her hands as if offering it to heaven. On the globe was the word “France,” and our Lady explained that the globe represented the whole world, but especially France. The times were difficult in France then, especially for the poor who were unemployed and often refugees from the many wars of the time. France was first to experience many of those troubles which ultimately reached many other parts of the world and are even present today. Streaming from rings on Mary's fingers as she held the globe were many rays of light. Mary explained that the rays symbolize the graces she obtains for those who ask for them. However, some of the gems on the rings were dark, and Mary explained that the rays and graces were available but did not come because no one had asked for them.
The Third Apparition and the Miraculous Medal
The vision then changed to show our Lady standing on a globe with her arms now outstretched and with the dazzling rays of light still streaming from her fingers. Framing the figure was an inscription: O Mary, conceived without sin, pray for us who have recourse to thee.
Then Mary spoke to Catherine: “Have a medal struck upon this model. Those who wear it will receive great graces, especially if they wear it around the neck.” Catherine explained the entire series of apparitions to her confessor, and she worked through him to carry out Mary’s instructions. She did not reveal that she received the Medal until soon before her death 47 years later.
With approval of the Church, the first Medals were made in 1832 and were distributed in Paris. Almost immediately the blessings that Mary had promised began to shower down on those who wore her Medal. The devotion spread like wildfire. Marvels of grace and health, peace and prosperity, following in its wake. Before long people were calling it the “Miraculous” Medal. And in 1836, a Canonical inquiry undertaken at Paris declared the apparitions to be genuine.
There is no superstition, nothing of magic, connected with the Miraculous Medal. The Miraculous Medal is not a “good-luck charm”. Rather, it is a great testimony to faith and the power of trusting prayer. Its greatest miracles are those of patience, forgiveness, repentance, and faith. God uses a Medal, not as a sacrament, but as an agent, an instrument, in bringing to pass certain marvelous results. “The weak things of this earth hath God chosen to confound the strong.”
When our Blessed Mother gave the design of the medal to Saint Catherine Labouré she said, “Now it must be given to the whole world and to every person.” 
Should you be experiencing difficulty in life, ask for Mary and her Miraculous Medals intercession. It will never fail the humble in heart. Mary will never fail you. She is here for us always.
Many people will say that they won't pray to the Blessed Mother because they are not Christian. Remember, the Blessed Mother was not Christian either. I believe she was Jewish. Don't allow religion to keep you from this beautiful intercession with Mary.
Here is also part of a story from Fr. John Hardon
"In the fall of 1948, the year after my ordination, I was in what we call the Tertianship. This is a third year of Novitiate before taking final vows.
In October of that year, a Vincentian priest came to speak to us young Jesuit priests. He encouraged us to obtain faculties, as they are called, to enroll people in the Confraternity of the Miraculous Medal. Among other things, he said, "Fathers, the Miraculous Medal works. Miracles have been performed by Our Lady through the Miraculous Medal."
I was not impressed by what the Vincentian priest was telling. I was not the medal-wearing kind of person and I certainly did not have a Miraculous Medal. But I thought to myself, "It does not cost anything." So I put my name down to get a four page leaflet from the Vincentians, with the then-Latin formula for blessing Miraculous Medals and enrolling people in the Confraternity of the Miraculous Medal. About two weeks later, I got the leaflet for blessing and enrollment, put it into my office book and forgot about it.
In February of the next year, I was sent to assist the chaplain of St. Alexis Hospital in Cleveland, Ohio. I was to be there helping the regular chaplain for two weeks.
Each morning I received a list of all the patients admitted into the hospital that day. There were so many Catholics admitted that I could not visit them all as soon as they came.
Among the patients admitted was a boy about nine years old. He had been sled-riding down hill, lost control of the sled and ran into a tree head-on. He fractured his skull and X-rays showed he had suffered severe brain damage.
When I finally got to visit his room at the hospital, he had been in a coma for ten days, no speech, no voluntary movements of the body. His condition was such that the only question was whether he would live. There was no question of recovering from what was diagnosed as permanent and inoperble brain damage.
After blessing the boy and consoling his parents, I was about to leave his hospital room. But then a thought came to me. "That Vincentian priest. He said, 'The Miraculous Medal works.' Now this will be a test of its alleged miraculous powers!"
I didn't have a Miraculous Medal of my own. And everyone I asked at the hospital also did not have one. But I persisted, and finally one of the nursing sisters on night duty found a Miraculous Medal.
What I found out was that you don't just bless the medal, you have to put it around a person's neck on a chain or ribbon. So the sister-nurse found a blue ribbon for the medal, which made me feel silly. What was I doing with medals and blue ribbons.
However, I blessed the medal and had the father hold the leaflet for investing a person in the Confraternity of the Miraculous Medal. I proceeded to recite the words of investiture. No sooner did I finish the prayer of enrolling the boy in the Confraternity than he opened his eyes for the first time in two weeks. He saw his mother and said, "Ma, I want some ice cream." He had been given only intravenous feeding.
This Experience Changed My Life
Then he proceeded to talk to his father and mother. After a few minutes of stunned silence, a doctor was called. The doctor examined the boy and told the parents they could give him something to eat.The next day began a series of tests on the boy's condition. X-rays showed the brain damage was gone.
Then still more tests. After three days, when all examinations showed there was complete restoration to health, the boy was released from the hospital.
This experience so changed my life that I have not been the same since. My faith in God, faith in His power to work miracles, was strengthened beyond description."
http://www.therealpresence.org/eucharst/mir/mirmedal.htm
