If you have an sd box, there is nothin you can do to get a better picture, it's a limitation of the box itself. You would need to switch to an HD box to be able to use the hdmi cable and gain the better picture quality.
if you bought an hdmi cable without checking te specs of your sd box, then you bought an unnecesaary cable.
the sd box depending on the model, may have rf, audio video composite, and s-video outputs. try using s-video if your box has it for the highest signal possible, otherwise rca composite is also good, if your using the rf cable and putting yoru tv on channel 3, that is the worst hookup possible.