Show/Hide #SharePoint columns in a List based on selected values in a drop-down list:

<script language="javascript" type="text/javascript">
$(document).ready(function() {
    showhide();
   $("select[title='FieldName']").change(function() { showhide(); });

});

function showhide()
{
   alert("inside function");// just to debug script , remove while move in production
    var thisVal =  $("select[title='FieldName']").val();
    alert(thisVal);// just to debug script , remove while move in production
    if (thisVal=="Somevalue")// your condition
    {
    alert("show all");// just to debug script , remove while move in production
     $('nobr:contains("FieldName")').closest('tr').show();

}
else
      {
     alert("inside else");// just to debug script , remove while move in production
     $('nobr:contains("FieldName")').closest('tr').hide();

}
</script>