A single article on fasting could go forever. Very long, at least. Instead, let’s consume this in bite size morsels (um, yeah, I’m currently fasting).
St. Gregory of Sinai gives these directions:
You who strive after salvation should be satisfied with one litra [3/4 lb.] of bread and three or four cups of water or wine a day, and a little of any other victuals which may be to hand. You must not let yourself eat to satiety. By thus eating all kinds of food you can both avoid boastfulness and avoid disdaining God’s creations which are most excellent; and you thank God for everything. Such is the reasoning of the wise! If you eat all the kinds of food at hand and drink a little wine, but doubt your salvation because of this, this is lack of faith and a disability of thought….
The measure of partaking of food that is free from sin and pleasing to God has three degrees: abstinence, adequacy and satiety. To abstain means to remain a little hungry after eating; to eat adequately means neither to feel hungry nor weighed down. But eating beyond satiety is the door to gluttony through which lust comes in. But you, firm in this knowledge, choose what is best for you, according to your powers, without violating the established rule: for the perfect, according to the apostle, whether they be satisfied or in hunger, are mighty in all ways” [Philippians 4:12-13] (p. 53)
And for specifics, he offers:
For those who are still forcing themselves, a pound of bread is sufficient, and three or four cups of water or wine, according to the day, is enough for one who wants to find God. As for the sweets that may be at hand, take a little of each but not to satiety, so as to escape conceit and not disdain the good creations of God, giving thanks to Him for everything. Such is the reasoning of the wise. For those who are weak in the faith, abstinence in food is very beneficial, for it is them that the apostle commands to eat greens [Romans 14:2], because they do not believe that they are protected by God.
There are three degrees of eating: abstinence, adequacy and satiety. Abstinence is when one is hungry after eating. Adequacy is when one is neither hungry nor weighed down. Satiety is when one is weighed down a little. To eat beyond satiety is the door to gluttony through which lust enters in. And so, examine all this and chose what is suited for your strength, without violating the rules. It is for the perfect, and this is according to the apostle, to go hungry and to be filled and in all things to be strong [Philippians 4:12-13] (pp. 77-79).
Fasting is an essential aspect of practicing the Orthodox life. You cannot be Orthodox and not fast (lots of links - lose yourself in this reading).
