First fasting is not a legalistic exercise. God does not particularly care how strictly you are able to observe a 5th century dietary code. Eating a Cheeseburger during Lent on a Friday is not a mortal sin (except possibly to your waistline). If you do eat one and happen to die before going to confession you do not need to plan on being buried in an asbestos suit. Point in fact very few Orthodox laymen keep the fast in its full rigor. I certainly have yet to keep the fast with anything close to perfection.
That said one should not just blow it off. Fasting is a spiritual discipline intended to stretch the body and help tame the passions. And it is a very important weapon in the spiritual warfare that we are engaged in more or less continuously until we die. So when you fall, don't give up. Pick yourself up, dust yourself off and get back on the wagon.
Also it serves no purpose to abstain from all manner of food and drink if we do not also give up our vices. In particular be wary of gossip. Your fast is your own business and no one else's (save God's and your confessor's). Likewise how others are keeping their fast is not your concern.
The Great Fast begins on Clean Monday (tomorrow) and continues until after Communion on Easter Sunday (Pascha). Technically Lent actually ends on the Saturday before Palm Sunday. But we continue to fast during Great and Holy Week. What follows is from the website of the Orthodox Church in America (OCA).
"What “precisely do the rules of fasting demand? Neither in ancient nor in modern times has there ever been exact uniformity, but most Orthodox authorities agree on the following rules:
The rule of xerophagy is relaxed on the following days:
- During the week between the Sunday of the Publican and the Pharisee and that of the Prodigal Son, there is a general dispensation from all fasting. Meat and animal product may be eaten even on Wednesday and Friday.
- In the following week…the usual fast is kept on Wednesday and Friday. Otherwise there is no special fasting.
- In the week before Lent, meat is forbidden, but eggs, cheese and other dairy products (as well as fish) may be eaten on all days, including Wednesday and Friday.
- On weekdays (Monday to Friday inclusive) during the seven weeks of Lent, there are restrictions both on the number of meals taken daily and on the types of food permitted; but when a meal is allowed, there is no fixed limitation on the quantity of food to be eaten.
- On weekdays in the first week, fasting is particularly severe. According to strict observance, in the course of the five initial days of Lent, only two meals are eaten, one on Wednesday and the other on Friday, in both cases after the Liturgy of the Presanctified. On the other three days, those who have the strength are encouraged to keep an absolute fast; those for whom this proves impracticable may eat on Tuesday and Thursday (but not, if possible, on Monday), in the evening after Vespers, when they may take bread and water, or perhaps tea or fruit-juice, but not a cooked meal. It should be added at once that in practice today these rules are commonly relaxed. At the meals on Wednesday and Friday xerophagy is prescribed. Literally this means ‘dry eating’. Strictly interpreted, it signifies that we may eat only vegetables cooked with water and salt, and also such things as fruit, nuts, bread and honey. In practice, octopus and shell-fish are also allowed on days of xerophagy; likewise vegetable margarine and corn or other vegetable oil, not made from olives. But the following categories of food are definitely excluded:
- animal products (cheese, milk, butter, eggs, lard, drippings);
- fish (i.e., fish with backbones);
- oil (i.e., olive oil) and wine (i.e., all alcoholic drinks).
- On weekdays (Monday to Friday inclusive) in the second, third, fourth, fifth and sixth weeks, one meal a day is permitted, to be taken in the afternoon following Vespers, and at this one meal xerophagy is to be observed.
- Holy Week. On the first three days there is one meal each day, with xerophagy; but some try to keep a complete fast on these days, or else they eat only uncooked food, as on the opening days of the first week. On Holy Thursday one meal is eaten, with wine and oil (i.e., olive oil). On Great Friday those who have the strength follow the practice of the early Church and keep a total fast. Those unable to do this may eat bread, with a little water, tea or fruit-juice, but not until sunset, or at any rate not until after the veneration of the [Plashchanitsa] at Vespers. On Holy Saturday there is in principle no meal, since according to the ancient practice after the end of the Liturgy of St. Basil the faithful remained in church for the reading of the Acts of the Apostles, and for their sustenance were given a little bread and dried fruit, with a cup of wine. If, as usually happens now, they return home for a meal, they may use wine but not oil; for on this one Saturday, alone among Saturdays of the year, olive oil is not permitted.
It has always been held that these rules of fasting should be relaxed in the case of anyone elderly or in poor health. In present-day practice, even for those in good health, the full strictness of the fast is usually mitigated…. On weekdays-except, perhaps, during the first week or Holy Week-it is now common to eat two cooked meals daily instead of one. From the second until the sixth week, many Orthodox use wine, and perhaps oil also, on Tuesdays and Thursdays, and less commonly on Mondays as well. Permission is often given to eat fish in these weeks. Personal factors need to be taken into account, as for example, the situation of an isolated Orthodox living in the same household as non-Orthodox, or obliged to take meals in a factory or school [lunchroom]. In cases of uncertainty each should seek the advice of his or her spiritual father [emphasis mine].”
- On Saturdays and Sundays in Lent, with the exception of Holy Saturday, two main meals may be taken in the usual way, around mid-day and in the evening, with wine and olive oil; but meat, animal products and fish are not allowed.
- On the Feast of the Annunciation (March 25) and Palm Sunday fish is permitted as well as wine and oil, but meat and animal products are not allowed….
- Wine and oil are permitted on the following days, if they fall on a weekday in the second, third, fourth, fifth or sixth week: [First and Second Finding of the Head of St. John the Baptist (Feb. 24), Repose of St. Raphael (Feb. 27), Holy Forty Martyrs of Sebaste (Mar. 9), Forefeast of the Annunciation (Mar. 24), Synaxis of the Archangel Gabriel (Mar. 26), Repose of St. Innocent (Mar. 31), Repose of St. Tikhon, Patriarch of Moscow (Apr. 7), Holy Greatmartyr and Victorybearer George (Apr. 23), Holy Apostle and Evangelist Mark (Apr. 25), as well as the Patronal Feast of the church or monastery].
- Wine and oil are also allowed on Wednesday and Thursday of the fifth week, because of the vigil for the Great Canon. Wine is allowed-and, according to some authorities, oil as well-on Friday in the same week, because of the vigil for the Akathist Hymn.
The following statement is extremely important to consider when we speak of fasting and fasting rules in the Church. “At all times it is essential to bear in mind that ‘you are not under the law but under grace’ (Rom. 6:14), and that ‘the letter kills, but the spirit gives life’ (2 Cor. 3:6). The rules of fasting, while they need to be taken seriously, are not to be interpreted with dour and pedantic legalism; ‘for the kingdom of God is not food and drink, but righteousness and peace and joy in the Holy Spirit’ (Rom. 14:17).”
The aforementioned guidelines are just that and as with much of the Church's discipline your mileage will vary. Expect some minor variations depending on ethnic and jurisdictional customs (i.e. The Greeks hold that the ban on oil applies only to olive oil, the Slavs tend to interpret that more broadly... For their part the Slavs often hold that beer is not wine and may be taken in moderation etc.). Also the above rules are those applicable to the Byzantine Rite. There are a small but growing number of Western Rite Orthodox Christians and their fasting discipline is different. See here and here for the fasting rules for Antiochian WR and ROCOR's WR respectively.