Type root to gain root access. Be careful while deleting/changing stuff as you would brick your lovely N900 

To delete a file type rm filename

To delete an empty folder type rm foldername 

To delete folder with files type rm -r foldername

To list files in folder type ls

To see hidden dot files in the folder type ls -a

To change folder type cd folder path

To see running processes type ps aux | less

To see running processes live type top

Here are some useful apt-get commands for N900 terminal :

apt-get update : Fetches updates from repositories

apt-get autoclean : Removes .deb files for packages that are no longer installed on your system

apt-get autoremove : Removes packages that were installed by other packages and are no longer needed

apt-get purge : Removes unnecessary packages and configuration files or something like that :) I'm no linux experts but when i run this i gain some space on rootfs

apt-get install <package name> :  Pulls and installs given package from repositories. Installed package will not be visible in Application manager!

apt-get remove <package_name>: Removes an installed package, leaving configuration files intact

apt-get purge <package_name>: Removes a package and the associated configuration files

See also Useful Commands for Nokia N900 terminal