Welcome home! One of the interesting things about Catholics is that, even if they have not been to Mass in decades, they still regard themselves as Catholic and would describe themselves as having a Catholic outlook on life.
Technically, the process of becoming active in the practice of your faith is quite simple, if there are no extenuating circumstances. It may only involve making your confession. Here, God reconciles us to himself and the Church. Confessing that one has chosen to be disobedient to the teachings and law of the Church is certainly an appropriate part of one's confession.
If the absence from Mass has been for some other reason, one that affects your status in the Church, then much more might be required. If, for example, there is a failed marriage and subsequent second marriage, then a case might need to be presented to the Bishop's Tribunal. We have experts who are ready to help you with this process. Please contact the office to request assistance at 972-727-1177.