The Rite of Christian Initation of Adults  (RCIA) seeks to prepare and form in faith two types of Adults for full initiation into the Catholic Church:

1) those who have never been baptized and
2) those who have been baptized, but not 'catechized' as Catholics

Unbaptized children who have passed their 7th birthday are received into the Church through this Rite as specifically adapted for this age group.

For those who are already Catholics, but have been inactive in the Church for some time, Saint Matthew many Adult Faith Formation (link) programs can assist you in relearning about Christ, your Church and living out the fullness of your Faith and sacramental life.