MuseScore exists for various different operating systems, including Windows and macOS, as well as many Linux distributions and several BSD variants. All are available from the MuseScore download page.
When you have downloaded the software, follow the installation instructions for your operation system:
The first time you start MuseScore after the installation, you will be asked to confirm your basic preferences:
The settings for language and keyboard layout are retrieved from your installation, and should normally not need to be changed. Just click Next to continue:
The Workspace page allows you to specify the default toolbars and palettes. Chose an option from the dropdown list. Then click Next to continue:
You will be now questioned if you wish to take the Tours. If you are new to MuseScore keep the 'Yes' option checked, and click Next to continue:
Finally click Finish to start MuseScore.