I have a p0420 code where should I start on 1998 Toyota Corolla

Before that I have a code that the auto zone guy said it was a oxygen secer I never replaced and just reset now it says p0420 bad cat. I runs great should start by cleaning maf or what lol

Asked by for the 1998 Toyota Corolla
Start by having a SHOP verify sensor and converter function with live data scan.