help making a security system...

yeah... i have seen everythng in tha playground are. but there is everything in parts.
like i know how to use the pir and buzzer seperately. but i dont know that how will it work collectively. what i mean to say is that i know how to make the buzzer work when the pir senses motion. but what i dont know is that how to display an alert on the lcd at the same time.