The interviewer is not asking for you to list your past jobs in chronological order with all of your responsibilities. They are only interested in a brief history, highlighting the relevant work you have done.
If you don't think you've done any work that is relevant to bartending, take some time to review your work history. Think about those transferable skills that you acquired from a job in customer service, for instance.
"I have three years' experience bartending at the competing pub, across the street from your establishment. I started as a bar assistant, then a backup bartender and now I am a senior bartender."
Entry Level answer example
"I have not worked as a bartender in the past; however, I have worked as a bar assistant for the past nine months. My duties as a bar assistant include ensuring the bar is fully stocked, running for ice, replenishing the garnishes, and general clean up. I am ready for this next step in my bar career."
Experienced answer example
"I have worked as a bartender for the past four years. Two years I spent in a local Irish pub, and two at a nightclub. It's been an incredibly lucrative career path for me, and I look forward to continuing this path at your establishment."