some guy or girl or group hacked my webpage, replaced the index.html file with one they edited to say "You have been paid a visit by us... thanks lanley and ^Mo-Shiza^."

I assume that those at the end are nicknames from mirc.

How do i stop this, and how did they do it?

Seeing as im posting in the newbie section, this obviously means im a newbie, so step by step instructions on prevention and offensive routines please.

Thanks!

Jeff