Our Lord says, “My church is not an ordinary house, My children. It is My castle and your King lives in it. There I am, night and day (in the Tabernacle). There, also is the Most Holy Trinity, there is My Mother, Mary Most Holy, there are Angels and Saints who praise Me constantly, but you, My children, give Me so little reverence. This upsets Me so much.”

Clothing

Modest clothing, both men and women, covering their flesh. For women - skirts below the knee, sleeves past the elbow. The Blessed Mother prefers feminine apparel rather than dressing like men. For men - long trousers and long sleeved shirts.

Attitude in Church

Genuflect on one knee humbly before the Tabernacle on entering and leaving the Church. Kneel on both knees (when possible) before the exposed Eucharist.
Refrain from holding conversations whilst inside the Church. Silence and reverence before God.
Hands folded and eyes downcast when approaching to receive Holy Communion.
Our Lady asks that women cover their heads while in Church. Our Lord wishes us to kneel and receive Holy Communion on our tongues.

Preparation

Attend confession at least once a month to be in a state of grace. Pray an act of contrition after the 'Prayer to the Lamb of God'. An Angel explained that we must prepare well before coming to Church. Prayer and reading the Mass, before Mass begins.
Offer petitions before Mass ego Relatives, the sick and Holy Souls.

Note: If we do not behave and pay attention during Holy Mass we will have to do reparation over and over after we die, sometimes for many years. St Catherine of Sienna explained this to me. She took me to the place where the Souls were doing penance for this irreverence. She explained that it may take centuries before the Soul is purified. IF NOBODY PRAYS FOR US.

After Mass

Stay a little after Mass ends to pray thanksgiving to Our Lord for the grace He has given us.
Be charitable, do not criticise and be judgemental as this offends God very much