Can a Catholic priest excommunicate someone for premarital sex? I was a Catholic. I did my sacraments according to the Catholic lifestyle. When I was born, I was baptized. Around age of 8 years, I got my First Holy Communion, and around 14 years of age, I went to confirmation classes and got confirmed. 

I got pregnant at age 17 and married outside the church, since my parents didn't approve of my condition or choice of the baby's father. Sex was preached as a mortal sin, and without confession, I would go to straight to Hell if I died. 

My parents disowned me and so did the Catholic Church, the priest told me I would be excommunicated. I don't know that any form was filed, and my parents apologized for doing me wrong many years later, instead of having the baby and having him adopted. 

Am I excommunicated or is all I need to do to go to confession to return to good standing in the Church?
Talk to a Priest. As far as I know marrying outside the Church is invalid but does not incurr automatic excommunication. You would not be able to receive Holy Communion, and I suspect that you may be confusing the two 

Even if you are excommunicated your excommunication can be lifted. Again a chat with a Priest, or an appointment for Confession would  be the place to start.

It does not seem like your marriages were valid, a Catholic has to marry in the Church or receive a dispensation. This is good news because you can get a decree of nullity for your previous marriages and convalidate your current one. That, and a good Confession should allow you to receive Holy Communion, assuming that you are not excommunicated or that if you are it has been lifted. Again a chat with a Priest should point you in the right direction to get the process started.