Here are a few problems that you might have with files.

I'm trying to write a file, but I get a "permission denied" message.
You don't have write permission for the file. If you're the owner (or root) you can change the permissions; see "File ownership and permissions," above.
I'm trying to list a directory that I have write permission for, but I get a "permission denied" message.
You need to have read permission for a directory in order to list it, and execute permission in order to interact with the items below that directory. See "File ownership and permissions," above.
I'm having trouble with a file that has a space in its name.
The command interpreter, or shell, parses the command line and uses the space character to break the command into tokens. If your filename includes a space, you need to "quote" the space so that the shell knows you want a literal space. For more information, including other special characters that you need to watch for, see "Quoting special characters" in Using the Command Line.