Just a quick shout out for www.songu.com
In my opinion this is the best site I've come across for developing your song writing ability.
You have direct access to extremely knowledgeable successful songwriters who are passionate about what they and you do as well as a wealth of information written and presented in a manageable style.
Once you get into the way the site works and their methods of teaching/lessons, there's no end to the amount of learning to be had.
Obviously don't take my word for it, take a tour and decide for yourself.
If you do decide to sign up enter this promo code TEAMSU9565 in the Promotions box and you'll qualify for a first month discount.
Promotional Code: TEAMSU9565
Have fun