Software - Chords

I've had songs floating around my head for many years now. I write down the melody and lyrics, and then just store them away. I play piano/keyboard, and I have a good basic knowledge of music theory, but I'm in need of a software that will take me to the next step. That is, take me from the melody (and sometimes I have a basic harmony) to a real composition. I've looked a lot, but I don't have a clue as to which software or book would be best to start with. Again, I am a published poet and lyricist, but I would love to write and sing my own songs as well. Without some help, my songs sound too basic to market. Thanks for any advice, Kathy