The traditional posture for prayer and worship in the Orthodox Church is to stand. In “Orthodox countries” there are usually no pews in the churches. Chairs or benches on the side walls are reserved for those who need them, i.e. the elderly, infirm, “with child”, etc. In North America, some Orthodox faithful have introduced pews into their churches thus creating the artificial “need” to figure out when to sit and when to stand. Whether a church has few chairs or many, there are times when one should definitely stand:
The beginning of the service: “Blessed is the Kingdom...”
All Litanies—The Entrances: with the Gospel and later, the Chalice
The Creed, the Lord's Prayer
The Anaphora beginning with “Let us stand aright” through the Hymn to the Theotokos
The distribution of Holy Communion, i.e. the Body and Blood of Christ in our midst, through the end of the dismissal
As you can see, this leaves little time for sitting. Whatever parish you are in, when in doubt, stand in prayer—yet remaining sensitive to not drawing attention to oneself, or blocking other's participation in the service.