“Finally, while respecting various styles and different and highly praiseworthy traditions, I desire, in accordance with the request advanced by the Synod Fathers, that Gregorian chant be suitably esteemed and employed as the chant proper to the Roman liturgy.”
Pope Benedict XVI, Sacramentum Caritatis, 2007
Here are some helpful links to websites that have Gregorian Chant materials
The FSSP, Our Lady of Guadalupe Seminary has on line exercises to teach Gregorian Chant. By starting with the basics you can teach yourself to read and sing chant.
Salve Regina (Solemn Tone) FSSP
“Gregorian Chant has always been regarded as the supreme model for sacred music, so that it is fully legitimate to lay down the following rule: the more closely a composition for church approaches in its movement, inspiration and savor the Gregorian form, the more sacred and liturgical it becomes; and the more out of harmony it is with that supreme model, the less worthy it is of the temple. The ancient traditional Gregorian Chant must, therefore, in a large measure be restored to the functions of public worship, and the fact must be accepted by all that an ecclesiastical function loses none of its solemnity when accompanied by this music alone.”
Pope Pius X, Tra le Sollecitudini, 1903
Gregorian chant belongs in the Catholic liturgy today. The Vatican II document,
states that "the Church acknowledges Gregorian chant as specially suited to the Roman liturgy; therefore, other things being equal, it should be given pride of place in liturgical services."
Sacrosanctum Concilium on Sacred Music A talk by Dom Cassian Folosm
Schola is group of men who sing the propers of the mass (the propers are sung Gregorian chant) that are in the missal and change every Sunday.