Apex code to generate SOQL to fetch all fields of object

String objectName = 'Lead';
Schema.SObjectType objToken = Schema.getGlobalDescribe().get(objectName);
//find details about sobject
Schema.DescribeSObjectResult objDescribe=objToken.getdescribe();
system.debug('*********sobjectAPINamel:'+objDescribe.getName());
//finding all fields of Lead and generating query dynamically
Map<String, Schema.SObjectField> fieldMap = objDescribe.fields.getMap(); 
String query = 'Select  ';
string conid='00Qxxxxxxxxxx'; //you can hard code the LeadId or you can pass it through apex method
for(String ss:Fieldmap.keyset()){
	Schema.DescribeFieldResult fd=fieldMap.get(ss).getDescribe();
	system.debug('*********fieldAPINamel:'+fd.getName());
	query = query + fd.getName() + ',';
 }
query = query.subString(0,query.length() - 1); 
query = query + '  from ';
query = query + objDescribe.getName();
query = query +  ' where Id = \'';
query = query + conid + '\'';
system.debug('*********Lead  Query: ' + query);