Fast (or do not eat or drink) at least one hour before you receive the Holy Eucharist. The only exceptions are medicine, water, or unless you are sick and need to eat sooner. (Source: Canon 919 of the “Code of Canon Law”)
Come early, recollect yourself. Be on time for Mass. Habitual lateness for the Mass is improper and disruptive.
Dress properly. This shows proper respect for God and others. Always wear clothes that are modest, and, if possible, all things being equal, clean and the nicest clothes you have (your "Sunday best").
Sick? If you are sick, stay at home. You may watch the Mass online.
Use the washroom. It is better to do it at this point rather than during Mass.
Cross yourself with the Holy Water. When you enter the Church, cross yourself with Holy Water and thank God for the grace given to you at Baptism.
Genuflect toward the Tabernacle, where Christ is present. By allowing our right knee to hit the floor, we acknowledge He is our Lord and God.
If you cannot genuflect, then a bow is sufficient.
During Mass, if you pass in front of the altar or tabernacle, bow reverently.
Take off your hat or headgear. Women may wear a veil as a sign of one's desire to humble oneself before God.
Keep silent. Avoid unnecessary talk. Keep necessary conversation to a very low whisper.
Don’t eat. Do not bring food or drinks into the church. The only exceptions are taking your medicine or discreetly breastfeeding or giving a bottle to an infant. Water is acceptable if necessary. Don’t chew gum in church.
Do not use phones and electronic gadgets. Put them to silent mode or turn them off. Quietly leave the Church to attend to a call or emergency.
Actively participate. Join in all the appropriate gestures while at Mass for our participation to be in unison. We are parts of the Mystical Body of Christ. Sing and pray with the community. If you think your singing will distract others, or you do not quite know the hymn or prayers, a soft tone is fine.
Prepare your offering. Share the blessings that you have received from God.
Respect one's space. If you have cough or colds, do not receive from the chalice. Respect people close to you. Others might not like to have their hand held or even be talked to while at mass.
This is done only by the priest when we pray or sing the Lord's prayer (the "Our Father") during a Mass. The priest stands in persona Christi, in the orans posture, speaking to God on our behalf.
The laity and everyone else should not do the orans posture during Mass.
Receive Communion properly. Show some reverence before, during, and after communion. Before receiving, you must recollect, pray, and prepare yourself. Know the other details here.
Bring your children to the Mass. Children under the age of reason (7 years old) are not required to attend Mass. It is okay to leave them at home (with proper care) if it makes things easier on you or if they are particularly cranky, too energetic, or too noisy.
Sometimes, children make noise during Mass. If your child is distracting to others, bring your child to the Narthex, the “Cry Room,” or outside.
Children, who are old enough to pay some attention, are better off sitting "in front" with you so that they can watch closely what goes on in the Mass.
Children, who are old enough to read, can use the Sunday Missal so they can follow along.
Do not leave the Church before the Presider. The Mass ends officially when the priest leaves the Church. Join in the singing of the last song.
Afterwards, you may sit or kneel in silent prayer or simply gaze at Jesus in the Tabernacle. Remain thankful and joyful for the blessing of the Eucharist. Show respect in silence to those who choose to stay and pray.
Leave quietly. Genuflect toward the Tabernacle. Cross yourself with holy water.