The following are commonly used commands to acess files and directories:
filesystem - An overview of the unix filesystem.
files - An overview of unix files.

ls - List files.
cp - Copy files.
mv - Move (rename) files.
rm - Remove (delete) files.
chmod - Set the file permissions on a file or directory.

cd - Change current working directory.
pwd - Print your current working directory.
mkdir - Make a new directory.
rmdir - Remove a directory (must be empty first).

cat - Print files to screen.
more - Print files to screen, one page at a time.
less - Like more but better.
head - Print the first few lines of a file.
tail - Print the last few lines of a file.