Do check the new manual that has been started in the Wiki as well as the tutorials page.

MusE -- Short Reference

Arranger manage tracks and parts
Pianoroll Editor record and edit single events in pianoroll notation
Drum Editor record and edit single events in drum notation
List Editor edit single events in list notation
Score Editor record and edit single events in score notation
Transport Panel navigate in time domain, start/stop sequencer
Files files and filetypes used by MusE

MusE -- Getting Started

Create New Song how to create a new song
Record Events how to enter notes into your new song
Step Recording record a song using the piano keyboard in the pianoroll editor