You can use Hookpad for free right in the browser (there is nothing to install). Go here to try Hookpad or click the getting started link at the bottom of the page.
Why Hookpad? The Problem With Music Composition Software
In existing software for writing music, beginners often don’t know where to start. It can be really intimidating. Even for experts, existing tools lack awareness of how music is structured.
Hookpad solves these issues by building in concepts from music theory to create a tool for writing songs that is more approachable for beginners and more useful for experienced songwriters.