OR…you can just confess the sin and move on.
The OT and the NT give lots of specific examples of things that are forbidden and things that we should do. So, the first attempt at explaining what is a sin is to look at those things.
On a second go-around, there are things that are not covered in the Bible directly, and the Church has provided additional guidance in the Catechism of the Catholic Church.
The universal call to holiness tells us to go beyond the letter of these commands, and the commandments can be summarized as love God and love our neighbor. Do not do anything contrary to to that. We are COMMANDED to love – it’s not an emotion, but it is manifested in our actions.
Jesus said something about giving a drink of water to someone who is in need is something that we do for him. I don’t think that we are supposed to stop there, at handing out cups of water. Rather, I think this says that everyone has something to give, even the poorest, even if it’s just a drink of water.
We need to look at opportunities every day and every place we go, to do good and avoid evil.
It’s a practical question to ask, if we have committed a mortal sin which prevents us from receiving the Eucharist. Be prepared for confession, with a list of sins and the number of times each has been committed. Go in there, read your list and “get it over.” The priest does not often have a lot of time for counseling in the confessional, especially if there is a line of people waiting.