You need a startup script - the format of which depends on your operating
system, not Sage. Sage itself has no way to start at boot.
Something like:
su - sageserv /local-bins/start-sage
in a startup script will run the script 'start-sage' as a user 'sageserv'
then the script should have something like:
nohup /homes/sageserv/sage-4.4.2.alpha0-t2.math/sage
/homes/sageserv/notebook.py >> /homes/sageserv/logfile.out&
which will start sage, while reading notebook.py
Exactly how you do it will depend on operating system.
That's how I did it on Solaris, though the correct way to do it on Solaris
would be using the Service Management Facility (SWF). Your system might use
something like that - I'm not a linux guru
Dave