Do you want the trailer to play as a Flash intro page or have, for example, and inserted YouTube video on the Homepage of your website?
We are definitely going to put a creative trailer together this year too. I'd say take the general theme from your haunt, film local areas, buildings, etc, that incorporate the "creepiness" into the haunt and then throw in a bunch of video special effects. And if you have past footage of actor scares, scenery, anything along those lines would pull it all together too. I've definitely seen some awesome intro videos on other haunter's sites.
My buddy and I used to make a bunch of short films with our summer cars, offroading adventures, etc. We used my Mac computer using iMovie & iDVD. Very easy to use and you'd be surprised what a good video you can have in the end even if you've never done it before.
First and foremost you need the video recorder. 720 or 1080 HD is definitely the way to go.