Read something good. I recommend Theology of the Body for Beginners, by Christopher West. It’ll help especially in regards to your relationship with your girlfriend. Regardless of your different faiths, take time to pray to God together, asking him for his blessings. On that note, start to pray more.
Read about the Mass, why we do the things we do at Mass. Follow along and learn about the sacraments. Do you understand the “Real Presence” means? Even a basic understanding compels one to seek out more understanding.
Most importantly, go to Confession. Think of all the things you have done wrong in the eyes of God. Have you had sex? Have you been respectful of your parents? Do you attend Mass every Sunday? Why not? Is God first in your life?
Confession is not just a “slate-cleaner.” It’s an opportunity to redirect your life towards God. Get in the habit of going once or twice a month. This is the quickest way to becoming a better Catholic.
Talk to a priest!!! A good priest can truly make the most out of a Catholic. Be humble and admit your ignorance. If you truly want to learn about the faith, realize that the Truths of the faith aren’t easy, but once appreciated and accepted, they instill great joy in the hearts of believers.
If you would like to talk more, just PM. or continue the conversation on this thread. God bless! God knows we need all the faithful laity we can get to soldier on for Him.