[Discuss] Port Scanning

[Daniel M Gessel]

This thread makes me want to ask:

As an amateur (and neophyte) sys-admin, what should I be doing to check 
for vulnerabilities and attacks? My brother runs a publicly visible 
server, but I'm not familiar with the tools he uses and when I ask him, 
it all goes over my head!

Is there a guide/book/website that might help me keep my systems from 
being easily attacked?  I run Debian (or RPi OS, which is Debian 
derived) on everything.

Thanks,

Dan