How do I make the dropdown required by not allowing the "Please Select Option" as selected answer

Here's the generic function that will do that. It returns a boolean which can be used later on to decide whether to submit the file or not:

function validateRequiredFields(doc) {
	var emptyFields = [];
	for (var i=0; i<doc.numFields; i++) {
		 var f = doc.getField(doc.getNthFieldName(i));
		 if (f!=null && f.type!="button" && f.required && f.valueAsString==f.defaultValue) {
			emptyFields.push(f.name);
		}
	}

	if (emptyFields.length>0) {
		 app.alert("You must fill in the following fields:\n" + emptyFields.join("\n"));
		 return false;
	}
	return true;
}