Today is Ash Wednesday, the beginning of the Season of Lent, or what the Church calls in Her Latin tongue Quadregesima (The Forty Days). Lent is the great season of penance, in which the Church calls Her children to the remembrance of their sins, to contrition for them, and to much penance to repair their damage. Lent is also a season of mortality, as the Liturgy of Ash Wednesday has the distribution of ashes, in which the priest marks the foreheads of his parishioners with ashes in the form of a cross, whilst saying the words: From dust thou art, and unto dust shalt thou return, or, Repent, and believe in the Gospel.
The Season of Lent is also the great season of fasting, more so in the past than at present. Whether for good or ill, the Church's fasting rules have been relaxed to such a point that they are virtually non-existent. However, Ash Wednesday and Good Friday still remain days of fasting and abstinence which Canon Law binds the Faithful to observe.
Remember the purpose of Lent: to accompany Our Lord in His Passion. It's one thing to fast, but we must also be in a state of total detestation of our sins. We must also meditate frequently on the Passion of Our Saviour, and excite ourselves to relieve the suffering of Our Redeemer, as well as Our Lady. Remember, She also suffered for us. She suffered more than any other person, save Our Lord. Her greatest sorrows were that She could not 1) comfort Her Son in His Passion, 2) join Him in His Passion, 3) forget that even as His Passion countless souls would still be damned through their own sins. Because She perfectly united Herself to Our Lord in His suffering, and because She did much for our salvation, She is rightly called Co-Redemptrix. And the greatest suffering of Our Lord, after His crucifixion, is that He couldn't comfort His Mother in Her suffering. Therefore, He invites us to bring comfort to His Mother and ours, at the same time bringing comfort to Himself. For even in His most bitter Passion, He thought always of His dear Mother, and we must do the same.
Fasting: The primary and most typical practise during Lent is to fast. Fasting is defined as the limiting of the intake of food for a period of time in order to 1) subject the flesh to penance, 2) focus more on the spiritual life, 3) grow closer to God.
There are several types of fasting in the Church, including the Mediaeval/Black Fast, in which Catholics fasted until 3pm, after which they only had one meal only, and abstained all of Lent from flesh-meat, eggs and dairy products. This type of fast is still observed by Eastern Catholics and Orthodox. It is also becoming a practise amongst Latin Catholics again.
The modern form of fasting allows of one main meal and two small collations in the day that shouldn't equal the main meal. Eggs and diary products are permitted. Flesh-meat (or red meat) is allowed only at the main meal, except on Ash Wednesday, Good Friday and all Fridays of Lent when meat is forbidden.
St. Thomas Aquinas states: The Council of Chalons says: "During Lent those are by no means to be credited with fasting who eat before the celebration of the office of Vespers," which in the Lenten season is said after the ninth hour. Therefore we ought to fast until the ninth hour. However, the Angelic Doctor goes on to say: If however this were to prove a heavy burden to a man on account of sickness, age, or some similar reason, he should be dispensed from fasting, or be allowed to forestall the hour by a little.
Fasting is not binding on those under 18 or over 60, or for the pregnant or those with medical conditions that require medication.
I have also noticed that certain Traditionalists try to shame Catholics into thinking that not keeping a strict Black Fast is somehow wrong. St. Thomas states that: As to the use of the latter things in other fasts the custom varies among different people, and each person is bound to conform to that custom which is in vogue with those among whom he is dwelling. Hence Jerome says [Augustine, De Lib. Arb. iii, 18; cf. De Nat. et Grat. lxvii]: "Let each province keep to its own practice, and look upon the commands of the elders as though they were the laws of the apostles." Besides, it is the Church who determines the proper rules of fasting, not individual Catholics, and at the present She leaves it to Her children to determine the right type of fasting appropriate for their circumstances. Do not let anyone bully you into thinking that because you only fast for a little time during the day, that that is somehow inferior. It isn't! If you can only fast for three hours, say, fast well during those three hours. For a little penance done well is much more pleasing to God than a big penance done badly. I myself cannot go too long without food, otherwise I start to go dizzy. I tried a Black Fast once, and it made me ill, so I don't even attempt to do it. I just use the modern fasting rules of one meal and two collations as a basic guide and work off that. I just do what I can, and try to do it well for Christ, and ask Our Lady to perfect it for me.
Ironically, fasting until 3pm ended in the 14th Century, when the Church relaxed the fasting rule until 12pm. In the 19th Century, due to the Industrial Revolution, Pope Leo XIII further relaxed the rules for men working in the mines and in factories, etc. As the Catholic Encyclopaedia states: ….owing to ever changing circumstances of time and place, the Church has gradually relaxed the severity of penitential requirements.... Traditionalists want to act as if the Church cannot relax the rules of fasting, when in fact She can. So, if you hear any Trads telling you that the only fast is a Black Fast, don't listen to them.
Abstinence: Another practise of Lent is to abstain, or "give up", something that is licit to us for the Lord's sake. A very common thing to give up during Lent is chocolate. But it can be anything, like TV, movies, games, etc.
Spiritual practises: Rather than giving something up, perhaps take something up, like the Divine Office, the daily Rosary, daily Stations of the Cross, the Seven Sorrows of Mary, spiritual reading on the Passion, daily mental prayer, daily meditation of the Scriptures. Some of these practises are also good outside of Lent too, so maybe taking them up during Lent may help you to keep them afterward.
Also remember not to take too much upon yourself, especially if you know that you are not going to keep to it. Just do what you can, no matter how little, and do it well. The purpose is not to do X amount during Lent, but to do something penitential as a means of repairing damage done by our sins. If you offer your penances to Our Lord though Our Lady, you can be sure that Our dear Mother will perfect what you have done, and make it more pleasing to God, no matter how small.