Develop A Songwriters Mind
Besides learning an instrument and a bit of music theory I would say one of the most important things to learn is to develop your mind into thinking like a songwriter.
This means listening out for lyrics hidden in conversations, on tv shows etc...
Hearing rhythms and melodies in every day life, like the sound of a car moving, hustle and bustle of moving people etc...
Then its a matter of keeping record of all your ideas in a notebook or on a portable audio recorder.
Check out http://www.world-of-songwriting.com/beginner-songwriting.html for more great tips!