According to the Catholic Church, does abortion need to be confessed, and is it a mortal or venial sin? I am not Catholic yet, but am attending RCIA and my first confession is next week. I had an abortion years ago. 

According to the official teaching of the Catholic Church:


Do I have to confess this? 
Is this a mortal or venial sin?
I'm going to disagree slightly with Andrew Leach on this one.

As far as excommunication: since you obviously weren't a member of the Church at the time, excommunication is obviously impossible. The information Andrew gives on excommunication for abortion is valuable, though; you should certainly be aware of just how seriously the Church takes this action.

As far as "Do I have to confess this?"&mdash;Canon 988, section 1, of the Code of Canon Law states:


  A member of the Christian faithful is obliged to confess in kind and number all grave sins committed after baptism and not yet remitted directly through the keys of the Church nor acknowledged in individual confession, of which the person has knowledge after diligent examination of conscience.


You have, thus, an obligation to confess the abortion if it is a mortal sin. If it is a venial sin, you are encouraged, but not strictly obliged, to confess it:


  Without being strictly necessary, confession of everyday faults (venial sins) is nevertheless strongly recommended by the Church.


(Catechism of the Catholic Church, paragraph 1458)

Similarly, the Roman Catechism (or Catechism of the Council of Trent) states:


  All mortal sins must be revealed to the priest. Venial sins, which do not separate us from the grace of God, and into which we frequently fall, although they may be usefully confessed, as the experience of the pious proves, may be omitted without sin, and expiated by a variety of other means.


So the question becomes whether the abortion was, at the time you arranged for it, a mortal sin.

For a sin to be mortal, three things are necessary (see Catechism paragraphs 1857&ndash;1860):


Grave matter. The sin must be a serious offense against God's love; often this is interpreted as "a direct violation of one of the Ten Commandments".
Consent of the will. A sin, particularly a mortal sin, cannot be committed by accident, negligence, or coercion. It must be something chosen deliberately.
Full knowledge. The sin must be committed in the knowledge of its sinfulness.


Procurement of an abortion (which is how we refer to the action of a woman who has an abortion), as Andrew Leach points out, is very definitely "grave matter"; it is, in the Church's teaching, arranging for a human being to be killed. So that box is checked.

I don't think there's enough information in the question to assert baldly that full consent of the will took place. It appears probable from the way the question is phrased, though not definite, that there was no physical coercion taking place; nor do we know anything about the mental or physical stress that you were going through. This might&mdash;and I stress might&mdash;mitigate the seriousness of the sin.

Finally comes the question of full knowledge. Unfortunately, simply stating that you didn't believe at the time that the fetus or embryo was a human being is not necessarily enough to demonstrate that you didn't have full knowledge of what was being done. The Church does teach that certain moral truths are "written on the heart", and that if a human being could conclude, simply using natural reason, that an act was wrong, then you should have known it. But the question of whether one can conclude from natural reason alone that abortion is wrong essentially turns into the question whether one can conclude using natural reason alone that the fetus/embryo/etc. is indeed a human; and this I think is a vexed question. Thus, absent some more pastorally relevant information which doesn't appear in the question, I don't know that there's enough information to conclude with certainty that your act was based on "full knowledge" as the Church defines it.

All the above considered, my conclusion is: The information given in the question is insufficient to enable one to conclude with certainty whether or not this sin was mortal. 

What does that imply about the necessity of confessing this sin? If the sin is not mortal, it is not necessary to confess it; but if the sin is mortal and one deliberately does not confess it, one abuses the Sacrament:


  So important is it that confession be entire that if the penitent confesses only some of his sins and wilfully neglects to accuse himself of others which should be confessed, he not only does not profit by his confession, but involves himself in new guilt. Such an enumeration of sins cannot be called sacramental confession; on the contrary, the penitent must repeat his confession, not omitting to accuse himself of having, under the semblance of confession, profaned the sanctity of the Sacrament. 


(Roman Catechism)

What to do then?

The point of the Sacrament of Reconciliation is to restore and renew one's relationship with God:


  It is called the sacrament of conversion because it makes sacramentally present Jesus' call to conversion, the first step in returning to the Father from whom one has strayed by sin. ... It is called the sacrament of Reconciliation, because it imparts to the sinner the love of God who reconciles.


(Catechism of the Catholic Church, paragraphs 1423&ndash;24)

As with any relationship, openness is indispensable. Thus, it is probably a good thing to confess anything you might suspect to be a mortal sin, and certainly anything that is weighing on your mind.