Start with items you read at http://hotwired.lycos.com/webmonkey/01/11/index1a_page5.html?tw=multimedia and consider taking it to some professional if need be.
http://www.film-to-video.com/tutorial.html is great to read before the above link for more ideas.
I'm really, really hoping someone can help me. We just got married and we videoed the ceremony. It was in a screened in porch with a rushing creek outside. At the time, it was wonderful, but now we're back, we cannot hear anything that was said during our ceremony due to the noise of the rushing creek!!!! Does anyone know of anything we can do to drown out the rushing creek and bring up the voices? I'm probably hoping for too much, but I thought I would check anyway. Such a dope for not thinking of this before we videoed it.