A good place to start is at the Renfrew Center, renfrewcenter.com (800-RENFREW). Renfrew was the first residential eating disorder center, and its comprehensive website has interactive quizzes and signs and symptoms to help you determine whether you have a problem or not.
On Something Fishy -- something-fishy.org -- you'll find, among other things, lively message boards on a wide range of topics, a treatment finder, and a news feed that updates daily with links to studies and articles.
Besides offering a wealth of links and pages, the National Eating Disorders Association (nationaleatingdisorders.org) includes an information and referral helpline at 800-931-2237. You can also download a guide to insurance issues -- especially how to fight to get the coverage you need for treatment -- by going to the Parent Toolkit part of the NEDA site.
The Harris Center at Massachusetts General Hospital (massgeneral.org/harriscenter/index.asp), has a list of national organizations, complete with links, where you can get more information on everything from nutrition to advocacy. You'll also find interesting stats on this site as well as a list of frequently asked questions.