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.