Someone who would like to receive Holy Eucharist must be Catholic. If he has a serious sin on his conscience, he must first make a confession. Before approaching the altar, one should be reconciled with his neighbors. [1389, 1417]
Until a few years ago, the practice was to eat nothing for at least three hours before Mass; this was how people prepared to encounter Christ in Holy Communion. Today the Church requires at least one hour of fasting. Another sign of reverence is to wear one’s finest clothing—after all, we have a rendezvous with the Lord of the world.
From YOUCAT, page 132-133