For that high-level question, let's see if we can give you a high-level answer...
* Become familiar with linux commands. df and fsck are good ones to know about.
* Make friends with the /etc directory where the configuration information is stored.
* Know how to edit /etc/fstab
* If a RAID is involved, know how to insert the modules or prepare the kernel
* Know how to read and access /var/log
* If you have to rescue data, know how tar works. If you have to move it to a network volume, know how NFS and the like work.
* If there is a custom kernel being used, copy that too.
* Know how to use remote-control tools like VNC to investigate
* Know about dmesg and /proc
* Know the cat command
* Know how to bring up in single-user mode
* Build your own linux box and get your hands dirty.
I am sure there are others, but that is off the top of my head.