How do I cancel a job interview?

I am a seventeen year old looking for a summer job, and I'm not really having any luck. Monster.com led me to apply at "Vector Marketing," which I promptly filled out the application for. I guess it serves me right for not doing research, but it turns out that the job is a door-to-door knife salesman job, which is something that I am not looking for in a job. How can I respectfully decline the job interview scheduled for today?