Some useful commands (enter without apostrophes):
'id' tells you who you are (e.g. root or user)
'pwd' tells you where you are (e.g. /root or /home/user etc.)
'cd' changes to some other directory, e.g. cd /home/user
'cd -' changes back to the directory you were in before your last 'cd somewhere'
'ls' lists the files in the directory you are
'ls /home' ditto for /home
'ls -A' as above but will also list files and dirs starting with a "." (dot)
'mv' renames or moves a file, e.g. 'mv old new'
'cp' copy a file, e.g. 'cp file file.backup'
'df' to see the mounted filesystems (including your card(s)).
'df -h' the same, in human-readable format (i.e. kilobytes or megabytes)

And for those commands taking filenames as parameters: Use 'tab completion', e.g. 'mv some-f<tab>' will complete some-f to whatever starts with 'some-f', press tab twice to see all alternatives, then enter another character+tab to resolve ambiguity as necessary.
Tab completion also works for commands, e.g. you think the command starts with 'l' but you don't remember the rest: Enter 'l', then press tab a couple of times.

And lastly:
'command --help' (e.g. ls --help) will show the options.

EDIT: Added the 'df' command.