While all Christians are invited to received the blessed bread at the end of Liturgy (which is distinct from Communion), Communion in the Orthodox Church is for those who have been fully received into the Orthodox Church*, adhere to the Orthodox Faith of Christ, are not in a state of mortal sin, prepare themselves properly on a given week, and, unless there is a legitimate emergency, are in church in time to hear the Gospel.
According to Holy Scripture, we have to be careful to partake worthily of Holy Communion (1 Cor. 11), which means proper preparation:
1 Cor. 11
27 So then, whoever eats the bread or drinks the cup of the Lord in an unworthy manner will be guilty of sinning against the body and blood of the Lord. 28 Everyone ought to examine themselves before they eat of the bread and drink from the cup. 29 For those who eat and drink without discerning the body of Christ eat and drink judgment on themselves. 30 That is why many among you are weak and sick, and a number of you have fallen asleep. 31 But if we were more discerning with regard to ourselves, we would not come under such judgment. 32 Nevertheless, when we are judged in this way by the Lord, we are being disciplined so that we will not be finally condemned with the world.
1. In order to partake of Communion, one must first have put on Christ through Baptism (Gal 3.27), and receive the gift of the Holy Spirit in the Mystery of Chrismation (Acts 2.38).* An adult or youth of age must be an Orthodox Christian in good standing who lives the life in Christ; a Christian life of repentance, adhering to the apostolic Faith that He gave for the life of the world. This Faith is summarize in the Symbol of the Faith, that is, the Creed. It involves daily giving one's life to God, talking to God (praying), temperence, and acts and words of kindness and charity toward others, and making things right with those whom we have wronged and asking forgiveness.
2. The basic expectations are, in summary, a respect for Christ in the Eucharist and an adherence to the Catholic and Apostolic Orthodox Christian Faith. This means coming regularly to Liturgy and showing up in time for the Gospel, for we receive Christ in His Gospel before we receive Him in His Sacrament as a necessary requirement, the exceptions being those who are coming from work, are ill, pregnant, nursing a newborn, or have a legitimate emergency.
3. Periodic Confession. Orthodox Christians above the age of responsibility partake of Confession when encountering grievous or repetative sins, and otherwise during the periods of repentance (Nativity preparatory fast, Apostles' fast, August fast 14 days until Dormition), and especially Great Lent/Holy Week. Children usually go through First Confession classes before partaking of confession for the first time, usually sometime between the ages of 8 and 10.
4. To make sure that, for those in good health, that the Body and Blood of Christ are the first things partaken of (eaten/drank) in a given morning or evening. This means, on an ordinary Sunday, eating or drinking nothing from about midnight prior (9 hours). Those who have health problems or physician required needs, however, or work through the night by necessity, have dispensation on this point.
One must abstain from Communion if he or she is in an unrepented state of mortal sin. Mortal sins include blasphemy, heresy, schism, apostasy, murder, living in sexual sins, stealing, abusing drugs or alcohol, conspiracy of coveting to gain at the expense of another or what belongs to another, and related sins.
*For those who are baptized Christians but have not received the sacrament of Chrismation (the seal of the gift of the Holy Spirit by being anointed with Chrism), this involves undergoing a learning process, confessing the Faith from the heart, then being chrismated, and thereafter regularly partaking of Communion. Those who are not baptized must become Christians (followers of Christ), undergoing learning, then baptized and chrismated, and then enter the life of regularly and preparedly partaking of Holy Communion.