JQuery determines which li is currently clicked

Use $(this).index() to get the index of li. Here is an example of a style substitution:

$("#aa li").click(function(){
$("#aa li").removeClass("class Name, multiple class Space out ");
$(this).addClass("class Name, multiple class Space out ");
if($(this).index()!=0){
$("#submenu").hide();
}else{
$("#submenu").show();
}
})