I struggled with this during my query process, too. I had a couple agents/publishers ask me the same thing and it freaked me out, too. I know NOTHING about marketing. Diddly squat. But you can't say that. In fact, I was lucky to have an hour long conversation with Christine Whithohn at Book Cents last month and she gave me a TON of invaluable advice about the industry. One thing she touched on was the "marketing question". She said NEVER say you have no experience in marketing or that you'll just do whatever you're told to do. MAKE SOMETHING UP, if you have to. Try to sound like you've got a plan, even if you don't. She said agents appreciate that much more than "I don't know". As the other poster said, you really need to establish a presence online. Don't just use your regular Facebook page, make an Author Page. Also, create an author website. I made mine for free using Wix.com. They let you create a page for free and they host it for free. I didn't have to pay a dime. Here's a link to the website I created, if you want an example: http://xtymorgan.wix...tinamorganbooks
A lot of writers are using Twitter to promote their books now, too. I personally can't stand Twitter, but if my publisher asks me to create an account, guess what I'm doing? The more present you are on the internet and social media, the better.
So, bottom line is, when asked to present a marketing plan (unless they want it in a separate document), add a paragraph to your query that says something along the lines of: "As to marketing this novel, I plan to use my website and social media to promote my book. I am also ready, willing and able to attend conferences, books signings and marketing events to further promote my novel." That doesn't sound that great, but it gives you an idea of something you can start with.
Hope this helps and good luck!