ANSWER:  The Virgin Mary! 

The Miraculous Medal, which is officially known as the Medal of the Immaculate Conception, was designed by the Blessed Mother, who appeared to St. Catherine LaBouré in 1830 and told her to have a medal struck with the image she showed her. 

Mary said those who wore it around their necks would receive great graces. 

The Association of the Miraculous Medal will send you a free medal, which you can ask a priest to bless.

Not sure how to pray the Rosary? The beautiful book, The Rosary Prayer by Prayer: How and why We Pray the Christ-Centered Rosary of the Blessed Mother, will take you bead by bead, page by page, prayer by prayer.