How did you manage to build a website without ever reading documentation or a readme file?
Ahem... sorry, we get a LOT of these exact same questions. Nothing personal.
JoomlaComment requires PHP version 5.
EDIT: OK, the Stickied thread about this error seems to have disappeared. I'll forgive you this time
I am seriously amazed at how many webserver companies still use PHP4, it's very insecure and version 5 has been out since 2004