Halloween is just a little more than a month away. If you haven't learned balancing ambient and flash yet, you still have a month to do it! :) Check out this related post: http://betterfamilyphotos.blogspot.com/2009/11/taking-halloween-pictures-part-2.html
Remember - in terms of balancing flash and ambient, the easiest scenario is when the subject is dark while the ambient is bright. That would allow you to set the ambient to the level you want, then use flash to illuminate the subject to the exposure you want, without either interfering with the other.
If you want to learn TTL Flash in general, check out: