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