username prompt from a table

i just want to ask how can someone prompt for a username before opening a module(based on a table, let's say clm_users) and then set what the users can do based on a specified record (PRIVELEGE) and if the user has a NULL expiry_date associated with them. for example

TABLE: clm_users

USERNAME PRIVILEGE EXPIRY_DATE
-------- ---------- -------------
SMITH LIMITED
BLACK ALL JUN-13-2007
PIPER ALL

what do i need to do? i know i needed parameters, triggers, procedures but i am having a problem on how can i base it on a specific record on a table. all i know is in the case of specifying to and from dates and base the usernames and passwords from a database...

please do help. thanks...