By default, the current version of Raspbian outputs only to HDMI. In order to allow the Raspberry Pi to output to the composite video (yellow plug), you need to open the file /boot/config.txt and comment out the line reading “hdmi_force_hotplug=1” by adding a “#” to the beginning of the line.
Once that line is commented out, unplug the HDMI output and the next time you restart the Pi it will use the composite output. The composite output might not look perfectly centered, might not fill the screen, or it might be too high resolution, in which case you can check out this tutorial http://learn.adafruit.com/using-a-mini-pal-ntsc-display-with-a-raspberry-pi/configure-and-test
showing how to set the resolution and the overscan (padding).
For now, power the LCD with bench power. Later we will get it running off USB power.