I am going to visit a Catholic Church. What is a communion and what is this rosary the pray to?

Just sit in the back, remain silent, and watch and listen. Do not go forward to receive the sacrament (communion). When the service is over, the priest will greet attendees as they leave - if you wish more answers than you'll get just by visiting, ask him if you can stop by later and talk directly to him. They get questions from the curious all the time, and he'll make the time to talk to you.