FIRST you get baptized first then confirmed, if on Easter Vigil; then both at the same day.
1855 Mortal sin destroys charity in the heart of man by a grave violation of God’s law; it turns man away from God, who is his ultimate end and his beatitude, by preferring an inferior good to him. (1395)
1857 For a sin to be mortal, three conditions must together be met: “Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent.”131
1858 Grave matter is specified by the Ten Commandments, corresponding to the answer of Jesus to the rich young man: “Do not kill, Do not commit adultery, Do not steal, Do not bear false witness, Do not defraud, Honor your father and your mother.”132 The gravity of sins is more or less great: murder is graver than theft. One must also take into account who is wronged: violence against parents is in itself graver than violence against a stranger. (2072, 2214)
1859 Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God’s law. It also implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart133 do not diminish, but rather increase, the voluntary character of a sin. (1734)
1860 Unintentional ignorance can diminish or even remove the imputability of a grave offense. But no one is deemed to be ignorant of the principles of the moral law, which are written in the conscience of every man. The promptings of feelings and passions can also diminish the voluntary and free character of the offense, as can external pressures or pathological disorders. Sin committed through malice, by deliberate choice of evil, is the gravest. (1735, 1767)
Basically what the Catechism of the Catholic Church means by this is you have to confess mortal sin: grave, full knowledge and consent
Means it must be serious, gravely harmful to persons
Means it must be known its a sin before you commit or during committing it.
Means it must be by your own will, not forced upon you.
Thru all of this your singular instance of impure thought is only mortal if you indulged in it furthering its impurity.
It is always good for to confess venial sins.
One thing to take from this is:
Confess everything just talk to your priest, if your unsure confess it.