Here are some of the resources I have found most useful for preparing to hike the Pacific Crest Trail.
Pacific Crest Trail Association – permits
Halfway Anywhere’s PCT page – tons of useful info, including surveys of PCT through-hikers
Guthook Guide – up-to-date trail information used extensively by thru-hikers. Cost is $30, as of March 2019.
PCT Water report – information about where there are reliable water sources, particularly important in the SoCal desert
USDA’s Natural Resources Conservation Service SNOTEL reports – for tracking snowpack, which can affect water availability and trail conditions in the Sierra