Try with :first
like
$('#wrapper ul:first').attr('id', 'customeId');
Or you can try with :eq(0)
like
$('#wrapper ul:eq(0)').attr('id', 'customeId');
Even(May be) you can try with .eq(0)
like
$('#wrapper ul').eq(0).attr('id', 'customeId');
Make sure that you have this script on DOM
ready like
$(document).ready(function(){
$('#wrapper ul:first').attr('id', 'customeId');
});
And please correct your html because it is breaking while closing ul