Sinclair ZX Spectrum +3 Help?

With my Spectrum, I have a RCA picture Lead (Yellow) plugged into the back of the computer, where it says 'TV'. The other end is hooked up to my TV. When I turn it on, the light comes on as does the one next to the floppy drive. No picture appears on the screen. Why doesn't the picture show up, and what can I do to fix it?do I need a particular floppy drive to boot.