Are you asking about software for melodies or for lyrics?
As a side, I use plain old NotePad to start songs. Then I use a program call Lyricist to help me keep organized.
Keep in mind, software isn't going to help anyone be creative. That comes from within the writer/melody maker. Software just helps you put it all together.
If you're just starting out, I would hold off on dropping big bucks for software until you are more familiar with the entire process of song writing and melody making. Your own brain is a pretty good place to start. Using the tools at hand, pen and paper, can accomplish quite a bit.
If you need help in putting your melody together, ask away!
Good luck!
**