MuseScore is the world's leading free and open-source software for writing and sharing music, with a user-friendly interface and immensely powerful features.

MuseScore is free to install on Windows, Mac, and Linux machines (desktops and laptops). 

Click here to download MuseScore

MuseScore for IOS and Android devices (phones and tablets) is available for a fee.  I don't recommend using this version at this time.