"Moral conscience, present at the heart of the person, enjoins him at the appropriate moment to do good and to avoid evil. It also judges particular choices, approving those that are good and denouncing those that are evil. It bears witness to the authority of truth in reference to the supreme Good to which the human person is drawn, and it welcomes the commandments. When he listens to his conscience, the prudent man can hear God speaking." CCC 1777
Return of the Prodigal Son by Murillo
The Formation of a Moral Conscience
Unfortunately, conscience may be one of the most misunderstood things among Catholics in this modern age. Many Catholics, as well as others, have come to understand conscience itself as a source of moral law—meaning that one simply discerns the moral law by looking within himself. Thus, “I am following my conscience,” has come to mean little more than one is following his subjective inclinations, which are themselves often formed by the prevailing culture.

Conscience is, however, as understood and taught by the Church, a judgment of reason. It uses the objective standards of the moral law to judge the morality of acts in specific circumstances. Every man, therefore, has a duty to form his conscience by immersing himself in the teachings of Holy Scripture and the Holy Catholic Church. He must actually form his conscience by the objective moral truth, which he must then use to judge the morality of acts in the specific circumstances of life. 
If you want to know what to think and do in the difficult circumstances of life, you must know what the Scriptures and the Church teach.
Works That Will Help You Form a Moral Conscience
The Sources of Christian Ethics by Servais Pinckaers, O.P. (A very important scholarly work on Christian ethics. This book covers a survey of Catholic moral theology from the Patristic era to the present, especially focusing on the period after the Council of Trent, and the problems posed by the Protestant Reformation and modern philosophy. This book is challenging but understandable by laymen, and it may be the best book available on analyzing how we came to our modern predicament in moral theology.)
Learning the Virtues: That Lead You to God  by Romano Guardini (A fresh look at the virtues that never change. This is one of those life-transforming books on understanding and living the virtues.)
An Introduction to Moral Theology  by William E. May (A very fine introduction to the Church's teaching on the moral life. An excellent guide for beginners to intermediates.)
The Spiritual Combat  by Dom Lorenzo Scupoli (A practical manual for living a holy moral life. The author analyzes various situations in life and advises how to cope with them, preserving a pure conscience and improving virtue. A classic.
Morality: The Catholic View  by Servais Pinckaers, O.P. (A brief and very readable introductory condensation of Pinckaer's more scholarly The Sources of Christian Ethics. The author clearly and concisely shows that Catholic morality is about fulfilling our longing for happiness, excellence, joy, and truth.)
Catholic Morality: A Course in Religion, Book III  by John Laux (A short and straightforward course on traditional Catholic morality written at a high school level. Although short, it is a great little introduction to the major terms and concepts in Catholic moral teaching.)
Why Humanae Vitae Was Right: A Reader by Janet Smith, ed. (A compelling defense of the Church's teaching on contraception by clearly explaining what contraception does to marriage. By various authors.)
By Man Shall His Blood Be Shed: A Catholic Defense of Capital Punishment  by Edward Feser, Joseph Bessette (Drawing upon the wealth of philosophical, scriptural, theological, and social scientific arguments, this book explains the Church's perenntial teaching that capital punishment can in prnciple be legitimate--not only to protect society from immediate physical danger, but also to administer retributive justice and to deter capital crimes.)