"It is natural to mankind to make sacrificial offerings to the Omnipotent God. Man is incited to do so by a natural instinct without any express command or special injunction..." -St. Thomas Aquinas
"When the Eucharist is being celebrated, the sanctuary is filled with countless angels who adore the divine victim immolated on the altar." - St. John Chrysostom
Mass is the central act of worship in the life of a Catholic. (General Instruction of the Roman Missal (GIRM), no. 16). In a very real way, every single Mass is heaven on earth. We meet our Lord there and such a profound truth requires a careful formation and preparation for that meeting. Below are a few basics every Catholic should keep in mind when attending Mass.

Dress and Appearance

Despite the casual nature of our culture, we must keep in mind that we come before Jesus at Mass. Modesty and appropriate attire are necessary, especially in Church out of reverence for the Blessed Sacrament. 
The following guidelines are not requirements of the Catholic Church for attendance at Mass. They are guidelines to the attire best calculated to enhance reverence in the presence of our Lord. Jeans and other casual attire are inappropriate for attendance at Mass. Men, women, boys and girls should wear Sunday dress clothes that are modest. Dresses are particularly appropriate for women and girls, while jackets and ties are particularly appropriate for men and boys. Chapel veils or mantillas are appropriate for women and girls.

Actions That Can Detract From Reverence at Mass

Lack of liturgical decorum.  This is simply engaging in subconscious and/or distracting acts and motions at Mass. For example, while the orans position (the laity praying with outstretched hands) has a long tradition in Jewish and even ancient Christian prayer life, there’s no precedent for Catholic laity assuming the orans position in the Mass. The U.S. Conference of Catholic Bishops has stated: “No position is prescribed in the present Sacramentary for an assembly gesture during the Lord’s Prayer.” Similarly, while the sign of the peace is a long-standing Catholic custom, there is no precedent for the distracting practice of moving about the Nave to speak to and shake hands with those at a distance. The USCCB has again made it clear that ". . . it is appropriate that each person, in a sober manner, offer the sign of peace only to those who are nearest." The practice of holding hands during the "Our Father" is another example of a lack of liturgical decorum. Although widespread in the United States, it is an illicit addition to the liturgy. The official publication of the Sacred Congregation for the Sacraments and Divine Worship, Notitiae (11 [1975] 226), states the practice "must be repudiated . . . it is a liturgical gesture introduced spontaneously but on a personal initiative; it is not in the rubrics." 
Noise in Church.  Sacred silence is to be observed in a Catholic Church. The U.S. Conference of Catholic Bishops (USCCB) states: "Even before the celebration itself, it is a praiseworthy practice for silence to be observed in the church, in the sacristy, in the vesting room, and in adjacent areas, so that all may dispose themselves to carry out the sacred celebration in a devout and fitting manner."
A Few Helpful Resources
Failure to know and practice proper posture and gestures.  Everybody at Mass is supposed to be uniform in standing, sitting, and kneeling (GIRM 20), and there are universal rules about it. In this country you are still required to kneel during the Consecration, from after the end of the Sanctus until the Great Amen, even if there aren't any kneelers (GIRM 21; Appendix to the General Instruction 21). You are required to bow or kneel at the words "by the power of the Holy Spirit" in the Creed (GIRM 98). You are required to genuflect whenever you pass the Eucharist, whether it's in the tabernacle or publicly exposed except when in procession (GIRM 233; CB 71). After Communion you are free to stand, sit, or kneel as you choose.
"There is nothing so great as the Eucharist. If God had something more precious, He would have given it to us." - Saint Jean Vianney