Like I said, the top music producers in the world are using technology to help them write songs.
So why not you?
Some apps I have listed here. Please feel free to comment and leave your own ideas of good apps on the market.
LOOPY HD - Is one of my favourites. It is good for get ideas down and you can beatbox your own beats as well
Jimmy Fallon and Billy Fallon using the Loopy HD
Plus - A good you tube video on Beatboxing if you don't know how to.
I hope that is a good starting point. Add a comment for any that you know of to be added in part 2
Keep it simple with your song, most songs are based on four chords, and often the same four chords. The video at the end demonstrates that.
The difference between the songs is mainly the melody and the rhythm
Once you have your chord structure and a rhythm you like, you can start on you melody. You can either hum over the top a tune that may fit well or have a friend try to improvise along with the chords. You can even use apps such as Loopy HD http://loopyapp.com/