You need to be more specific.
Personally I think the songs you write should be completely your own, but if it's just for things like knowing which chords are in which key, or which chord progressions sound good (which you should be able to do on your own), then there are a lot of resources on the net for that, and I'm sure a lot of software too...
If you're talking about writing songs down, like a notation program, there is a free one called PowerTab at
www.power-tab.net or Guitar Pro.
If you're talking about software that helps you come up with melodies/riffs, that's something you should do on your own.