There are a ton of songwriting books, classes, and other resources available out there. The way I would approach it is first listen to song writers that you truly love and see what they do. Along with this, yes; a good song writing book doesn't hurt if this concept is new to you. Here are a few I stumbled across;

http://www.amazon.com/Songwriting-Sourcebook-Chords-Great-Songs/dp/0879307498/ref=sr_1_11?ie=UTF8&s=books&qid=1260405289&sr=8-11

http://www.amazon.com/Songwriting-Dummies-Jim-Peterik/dp/0764554042/ref=sr_1_1?ie=UTF8&s=books&qid=1260405364&sr=8-1

As you progress, through this; thus begins the dreaded, but rewarding, process of trial and error. A great avenue to get feedback is by simply using this site; uploading your tunes through the forum. There are many users and instructors on this forum that are working professionals in the music business and are always happy to help and to provide feedback.

Best of luck on your journey and keep us posted along the way......