There are a bundle of available e-mail form scripts at HotScripts, in this section:
Of course, finding the right one is the trick, but downloading any of the very simple ones will probably give you a pretty good idea of ways to do what you're looking for. Check out the one on this page as a very simple script:
I was actually looking to do something similar, although in my case I wanted the web form to produce a specially formatted message for me, so I'm probably going to be writing one myself using one of these scripts as an example.
One important thing to keep in mind: If the script gets the "to" address from the form (even as a hidden variable), it's a huge potential security hole for spammers to take advantage of, so it's probably better going with a thoroughly secured existing script unless you really know what you're doing.
I believe, however, that if you hardcode the "to" e-mail address in the php script there's not much that can be done to exploit it. Someone who knows more than I do like to chime in on that? I'd really like to know so I'm not opening up DH and myself to nasty spam exploits.