Forward of a website.com to a www.website.com ?

[BoogieAgogo]

Hi,

I know this seems like a very little question, but I can't find the
answer:

  How do I get a domain to forward to the WWW. when a user does not
type it?  ( Example, if you type yahoo.com it will forward to
www.yahoo.com )

I know it's not that big of a deal, but I have noticed on my websites
that my Google Pagerank does not show up for the non WWW address.  I'm
not sure if it affects pagerank, but I would just prefer to be on the
safe side.  

I use Plesk 7.5/linux.

Thanks!!