What do I do if I was fired and my boss is threatening to file for grand larceny against me?

I've worked at my job for the past year and I have never had a problem. My boss used a POS system that keeps track of all purchases and monitors inventory. He is claiming that I have over time stolen over $1500 from his business. On my shifts, the cash register was always balanced and money was never short. He claims that I gave away products, didn't ring up sales, and took the money for myself. He has video cameras and he told me he has seen me take money out of the register at night and put it in my pocket. I often get tips through the day and take them out during closing. But even after I take the money the cash count was always right and had NEVER been short. I'm only 19 and I'm just very nervous. I don't want any criminal charges on my record. I'm just wondering if there is any possible evidence against me being there are records of cash counts always being correct and never a problem. I also am working under the table, getting paid in cash and have no contact with my employer.