Download MuseScore, to write songs and symphonies!
MuseScore is a WYSIWYG free program that will let you write a musical score on your computer, providing all the necessary tools for using musical notation. MuseScore incorporates even a sequencer and synthesizer to play your score. The Play Panel offers more controls over playback including tempo, starting position, and general volume.
Features: WYSIWYG, notes are entered on a “virtual note sheet” * Unlimited number of staves * Up to four voices per staff * Easy and fast note entry with mouse, keyboard or MIDI * Integrated sequencer and FluidSynth software synthesizer * Import and export of MusicXML and Standard MIDI Files
Here is a nice composition created by an anonymous 15 years old kid, using MuseScore: