If you are not a baptized Christian, then you cannot validly approach the other sacraments. Your baptism will wash away Original Sin plus all your personal sins since the time of your birth. You will receive First Holy Communion shortly thereafter with a clean conscience in a state of grace. After your initiation, you will begin to frequent the Sacrament of Penance. RCIA should prepare you adequately to approach this sacrament.
Right now, since (I assume, as an agnostic) you are unbaptized, it would not help you to approach the Sacrament of Penance or Eucharist or any other sacrament except Baptism. This is the gateway to all other sacraments. Your desire to be baptized is taken into account, should you unexpectedly die before it happens. You will be assured a Christian burial. In fact, if there is any time at all should you be sick or injured, someone, anyone, can baptize you in an emergency. So do not worry too much about your past or current sins. Just follow the direction of your priest and enjoy the mysteries of Christian life.