That is a pretty open question. Depends on a lot of things, what distro are you planning on using to begin with? What desktop? What browser? A really safe way would be to use Knoppix.
Hd Install HowTo - Knoppix Documentation Wiki
http://www.knoppix.net/wiki/Hd_Insta...f_installation
Install it choosing the Knoppix option, basically installs it in LiveCD mode, meaning things can't really be change much. Just make sure you set a good root password since users are automatically logged in.
Kde has a way to set it up in kiosk mode.
KDE Kiosk Mode | Linux Journal
http://www.linuxjournal.com/article/7718
Also depends alot on what browser you plan on making available to people.
There is a way to setup Firefox in Kiosk mode . As well as an extension.
FireFox (Firebird/Phoenix) Kiosk Browser
http://tln.lib.mi.us/~amutch/pro/phoenix/kiosk.htm
If all else fail simply run a LiveCD. Nothing to screw up or repair. If something does happen to get messed up just reboot and its fixed.
A little outdate but worth taking a look at is:
Kiosk HOWTO <author>Gene Wilburn, ITS Dept, Royal Ontario
http://www.faqs.org/docs/Linux-HOWTO/Kiosk-HOWTO.html
security checklist for a public internet kiosk?
