Im trying to nut out a check box validation

The goal-
the check box as default is unchecked and when checked validates a text area for text

Any ideas?

function checkText() // check box
if (document.Forders.card.checked == false)//true?
if( document.Forders.special.value.length ==0 ||document.Forders.special.value == "Enter your personal message here" )
alert ( "Please enter your personal message in the text area" );
return false;