When safari is opened I am told my Mac "is at serious virus" and gives me a number to call. I have been told to not call this number, but what should I do?

When safari is opened I am told my Mac "is at serious virus" and gives me a number to call. I have been told to not call this number, but what should I do? Safari is also running extremely slow because of whatever this is. I have been advised to not call the number that pops up so I don't get scammed, but I'm not sure what the best course of action is in this case.