Recieved write up accusing of Slander againest the business which was false. Legal Advice?

I received notice today that I have "slandered' the business I work at and received a final warning, which was my first one. The DM told the GM to investigate me and call other business and ask whether or not I had announced in a public setting the business would be closing. I know these accusations are false and would like to know what my legal rights are to follow up on.