Well, it depends. There should be a formation process in every parish for baptized Catholic adults who need to complete their Christian initiation with first communion and/or confirmation. There should also be a process to receive non-Catholic Christians, or unbaptized adults, into the Church, and it is called RCIA. It is imporant to make the distinction between the baptized and unbaptized, and the rites each of these groups celebrate along the way in this journey do keep that distinction.
that being said, as a practical matter, most adults seeking sacraments need instruction on the same topics–the Creed, that is the beliefs and teachings of the Church, so they can make an informed profession of faith; the sacraments and liturgy so they can understand what they are requesting and share in the worship of the Church; morality and the Christian life so they can make choices and decisions with an informed conscience and conform their lives to that of Christ; and Christian prayer and spirituality so they can grow closer to God and pray with all Christians, alive or dead, in heaven, on earth, or in purgatory, as part of the communion of saints.
so in most parishes you will probably be invited to join a class to prepare for your own sacraments, and be with people who fit the various descriptions above. You will, I hope, find as most others have done that making the journey together is very beneficial spiritually on many levels.
Please see your parish priest today to get started, this is the perfect time of year.
Welcome Home.
if you have already made first communion you need only make a confession going back to your last one (make an appointment if you think it will be a long one) and return to the sacraments, and then begin preparing for confirmation as soon as the class begins in your parish.
this presumes you never formally joined another religious body (if that is the case, tell your priest and he can guide you)
Explain the circumstances and they will be glad to help.