Because of the intimate connection of the Sacrament of Holy Communion to our life in Christ, we must be free of any grave or mortal sin before receiving it, as St. Paul explained in 1 Corinthians 11:27-29. Otherwise, as he warns, we receive the sacrament unworthily, and we "eateth and drinketh damnation" to ourselves.
If we are aware of having committed a mortal sin, we must participate in the Sacrament of Confession first. The Church sees the two sacraments as connected, and urges us, when we can, to join frequent Confession with frequent Communion.
In order to receive Communion, we must also abstain from food or drink (except for water and medicine) for one hour beforehand. (For more details on the Communion fast, see What Are the Rules for Fasting Before Communion?)
If we cannot receive Holy Communion physically, either because we cannot make it to Mass or because we need to go to Confession first, we can pray an Act of Spiritual Communion, in which we express our desire to be united with Christ and ask Him to come into our soul. A spiritual communion is not sacramental, but prayed devoutly, it can be a source of grace that can strengthen us until we can receive the Sacrament of Holy Communion once again.
Receiving Holy Communion worthily brings us graces that affect us both spiritually and physically. Spiritually, our souls become more united to Christ, both through the graces we receive and through the change in our actions that those graces effect. Frequent Communion increases our love for God and for our neighbor, which expresses itself in action, which makes us more like Christ.
Physically, frequent Communion relieves us of our passions. Priests and other spiritual directors who counsel those who are struggling with passions often urge frequent reception not only of the Sacrament of Confession but of the Sacrament of Holy Communion. By receiving Christ's Body and Blood, our own bodies are sanctified, and we grow in our likeness to Christ In fact, as Fr. John Hardon points out in his Modern Catholic Dictionary, the Church teaches that "A final effect of Communion is to remove the personal guilt of venial sins, and the temporal punishment [earthly and purgatorial] due to forgiven sins, whether venial or mortal."