The Appalachian Trail Conservancy is the official source for all things "A.T." Their website has a plethora of resources to plan a hike, volunteer opportunities, and just general hiking information.
Use this link to go to their web site.
Use this link to go to their web site.
The Keystone Trails Association protects, manages and maintains the trails in Pennsylvania. For information on the thousands of trail mails in Pennsylvania go to KTA's web site.
AppalachianTrail.com is another resource you may use for anything A.T. related. Check this resource out with this link.