Nginx on 10.7 with homebrew
If you type brew info nginx you should get answers to many of your questions.  In particular, you should see something like:

To have launchd start nginx at login:
    ln -sfv /usr/local/opt/nginx/*.plist ~/Library/LaunchAgents
Then to load nginx now:
    launchctl load ~/Library/LaunchAgents/homebrew.mxcl.nginx.plist


So I expect if you link the plist to /Library/LaunchDaemons instead of ~/Library/LaunchAgents it will load at boot.

I don't know what you want to change, but it's the same nginx as you use on Debian, so you can copy over your Debian nginx.conf if you want.  Also the brew package comes with a set of default/example configuration files installed under /usr/local/etc/nginx/.  

The default conf runs on port 8080, so you have to edit it to run on port 80, and yes it will conflict with Apache running on port 80 so you have to turn Apache off or put it on another port.  Apache config files are in /private/etc/apache2/ and you can turn it off via System Preferences -> Sharing -> Personal Web Sharing.  (Note that this option has been removed in Mountain Lion.  Mountain Lion still ships with Apache, but it is disabled by default.)

You can, of course, set it up however you want, but if you're looking for what's easy and Mac-like, just follow the Apache setup, which uses user and group _www and uses /Library/WebServer/Documents as document root and serves user content out of ~user/Sites.