The first thing you should know is that you never left the Church. Through your baptism, you always remained Catholic.
However, if you’ve been away from the Church for several years, it would be good to take a refresher course. For example, there is a course called Rite of Christian Initiation for Adults (RCIA). Although you are not preparing for sacraments of initiation, it would be a good place to become re-acquainted with the Church.
Also, if your parish offers a bible study, this would also be a good place to start.
See Catholics Come Home.