Hi ,
Selection screen will be called by default. So no need to call. But when user clicks on the execute button(F8) under START-OF-SELECTION you can call your screen using CALL SCREEN 1500. On this screen you can create Custom Container on top of it you can display the ALV grid container. Also in PBO of the same screen you can write you data retrieval logic & pass the same to your ALV Grid .
Thanks & Regards
Vishal