Put this proc in a file /packages/myfirstpackage/tcl/scheduled-init.tcl. Files in /tcl with the -init.tcl ending are sourced on server startup. This one executes my_proc every 60 seconds:
ad_schedule_proc 60 myfirstpackage::my_proc
This executes once a day, at midnight:
ad_schedule_proc \ -schedule_proc ns_schedule_daily \ [list 0 0] \ myfirstpackage::my_proc
See ad_schedule_proc for more information.