In it's simplest form there are two options if = INPUT and of= OUTPUT e.g.
dd if=/dev/zero of=/somefile
This would create an empty file full of zeroes and keep writing to it until you cancel. Just remember however you use DD, there is never any user confirmation so be very careful when typing paths becuase if you accidently put of=/ somefile instead of =/somefile you could end up trashing your filesystem by writing zeroes over it.
You can also specify byte sizes and the number of counts etc. Type "man dd" and you should have lots of syntax etc in there.