Choose a notation program that allows you to use musical notation to write your song. There are several programs available (see resources). They work very much like word processors, except that you write with music notation.
Open your notation software and begin constructing your song. On the "File" menu, choose "New" to begin a song and select the time signature and key for your song. Notation software typically will open a dialogue box to allow you to select these elements.
Add notes to your sheet music by using the indicated keyboard shortcut or by clicking the appropriate note value on the toolbar and clicking the position on the music staff where you want the note to appear.
Review your note choices by clicking the playback feature on the toolbar. Change any notes that don't sound right to you by clicking to select them, then using the delete key on your computer keyboard. Add new notes until your song sounds the way you want it to sound.
Print your song from the "File" menu. There's no need to purchase music staff paper. Notation software prints your song with the music staff in place.
Visit Noteflight, the only online music notation software. The website requires you to create an account, but there is no charge to use it. Click on "New Score" after you create your account. Choose "Private" to keep your music score viewable by you only and "Shared" if you want others to see it.
Edit the default sheet music to suit your needs. The default is the key of C with a 4/4 time signature. Click twice on any of these elements to change them; a floating palette will open. Select a new time signature or key signature from the palette, then click "OK" to change them.
Place notes on the music staff by selecting the note type you wish to use and moving your cursor over the music staff to the line or space you wish to place the note. Click the line or space to place the note. Add rests to the music staff in the same way.
Review your finished song from the "Play" menu. Select "Play from Start" to hear your song. Select new notes for any of the notes that don't sound good to you, then go to the "File" menu to save your finished song.
Print your song as sheet music from the "File" menu. If you want to export your song as a music file you can listen to, choose "Export" from the "File" menu and select the MIDI or WAV option.