After you set the date -- ideally about four weeks ahead of time, so you'll have plenty of time to plan -- you'll want to establish how many people can come. If the number is a firm 20, then make sure you and your teen are on the same page about this, and that it will be his or her responsibility to stick to that limit. Then mentally put in place a plan for how to handle uninvited guests, because you're bound to get a few. Teens may not be hip to cutesy Halloween-themed paper invites or Evites, so don't insist. Let your teen handle the invitation with the communication method of choice: probably e-mail or text.
Make sure guests know that the party has a specific start and an end time. Also, be sure to set some ground rules: no alcohol, no inappropriate costumes and no spin the bottle. Depending on how many guests you invite, you may want to enlist a few other parents to help with the chaperoning. You don't want to embarrass teens by breathing down their necks, but you do want to remain close enough that they know you're looking out for funny business and will shut it down immediately.