There are several first-time access points to the Resting Grounds which require different amounts of progress: In order to start collecting essence, you need to acquire the Dream Nail which can be found in the Resting Grounds.
In particular, this guide will give the basics of dream nail use to collect essence, the total (known) essence in the game, the locations where you collect essence, and the rewards the Seer gives for reaching certain essence amounts. This guide aims to cover details regarding essence collection in Hollow Knight.