I'm assuming that you are using a dinky lil' point and shoot? Well, you see the little flower button on your camera? That opens up the aperture and allows you to focus up close. Push da button!
Seriously though, do it. It might not make the background quite as blurry as you want, but it's a start, eh?
If you're using an SLR (Digital or analogue), switch to the aperture priority mode (Marked with an "A" on the control dial), and select the biggest aperture you can (Remember, 2.8 = big aperture, 11 = small aperture). Of course, you will be limited by your lens.