Why doesn't linux understand my commands
 

I bought redhat linux fedora for dummies, book and instalation DVD and carried out the instructions correctly (as far as i can tell).
everything seems ok when i first log on but when a open a terminal window, log on as root, and try to input a command as quoted in the book (eg iptables -L)i receive the message 'iptables command not konwn'. all commands seem to be replied with 'command not known'. I guess i must have messed up the installation some how. I've tried re-installing but the problem persists.
I'm new to linux and really want to get into this but i'm growing very frustrated.
please help!

This is because you're running those commands as an unpriviledged user and those specific ones need to be executed as root. The reason is that those commands (eg: iptables) are typically in root's $PATH.

So, either run these as root, or modify your $PATH so those directories are in it. I advise reading RUTE.

su will use the users environment not root's
su - will use roots environment.

Actually Fedora is very peculiar in this. Even you are root you can't use the commands reserved for root in any directory. You have to go to /sbin and then use these comands using ./iptables for example.

If you want to correct this bevahior, follow scuzzman advice and modify your $PATH and include /sbin.