If you were baptized Catholic, you are still Catholic. To come back to the Church, you need to make a good confession, and then start attending Mass and receiving the sacraments of Holy Communion and Confession regularly. (I am assuming that you have already made your first Confession and first Communion. If not, a priest can help you prepare for these sacraments, but you are still Catholic.)
Once baptized a Catholic, you are a Catholic for life. Baptism puts an indelible mark on your soul.
Any sins you committed can be forgiven in the Sacrament of Reconciliation (Confession, Penance). God’s mercy is without bounds. Isn’t that great?!
You received the Holy Spirit at Baptism. He remains with you throughout your life, unless you separate yourself from Him by serious sin. He is waiting eagerly to come back to anyone who repents and confesses their sins.
At Confirmation, the Holy Spirit comes to you in a fuller, more abundant way, preparing you to live as a faithful Christian adult, even in times of persecution. Talk to your Catholic pastor about helping you receive this wonderful Sacrament.
We in the Church have missed you; hurry back, and welcome!