Tax verification?? Why first time filing help

This is my first time filing I am 20 years old and reporting cash. I clean offices and get paid cash. I did taxes and everything was good till I got a letter that I need to verify my identity I tried over phone but didn’t go thru so they said I had to go into there office so I am. They told me to bring socials Id and birth certificate and my Schedule C form. What questions will they ask me while I’m there? I’m really nervous for some reason and want to be prepared. By the way I’m claiming my daugther. Her father claimed her last year ! Thank you’