Not everyone needs to work with music scores on their computers, but if you're someone who does enter, edit, or store sheet music electronically, you can choose among many free software options.
LilyPond, a command-line-based program, is the simplest and clearest way of managing your music. You work with LilyPond in two stages. First you describe (in a text file) what music you want to engrave, using note names, their durations, and various commands. Then you run the lilypond command on the file you created to produce a PDF file with your music.