After creating Object What happen???????

Actually after creating Object Like
                ClassA obj123 ;     //obj123 is Variable of ClassA
               obj123=new ClassA();    // but here obj123 is reference Object
  but after creating object with help of new keyword, then automatically memory location is allocated in Heap memory and reference of location is stored in Stack  memory
                       but My Question is :-->> which one(Heap/Stack) is executed first  (when do i call a method &&when do i execute my program????????)