5.1 can be achieved directly out of Logic Pro 7, but you'll need an interface capable of dealing with 6 channel audio and the 5.1 speaker rig itself.
If you haven't done much audio mixing before, attempting 5.1 will be very difficult, and Logic from basic precepts is a steep climb too.
Once you have the 5.1 channels as discrete (separate) audio files, you can use something like A-pack (which comes with DVD Studio Pro) to create the AC3 file that is actually played by the DVD player.
If you have FCP Studio, then you'll have Soundtrack, give that a try to start with. You can practice with Garageband, but not with a video component and only in stereo.
Have a look at
http://www.osxaudio.com for a bigger community view.