In case it isn't obvious, I would recommend using a bulk email service (I use Mailchimp) rather than DIY.

I did DIY for a while myself and it was a harrowing experience. I was always worried that my script would get caught in an infinite loop or expose everyone's email address somehow. It's much easier and I feel safer using a bulk service. Plus, they add analytics, easy image insertion, etc, etc, etc.

Finally, most bulk services will have hotwo or tutorial articles answering most of these questions. They know that we don't really know what we're doing, so they try to do a little handholding.