MuseScore -- Index -- Create New Score

Create New Score from Scratch
The menu entry Score New creates an empty new score. First you have to populate the empty sheet with instruments and then you can add empty measures. Next steps are usually: set clef, set key, set time signature.
Create New Score from Template
N popups the Template dialog. Select a template and click OK . This is the simplest way to create a new score. Note that template files are normal MuseScore files stored in the template folder.
Create New Score with the New Wizard