This is a great question with a simple answer. Many musicians associate the term "music theory" with a lot of boring unhelpful material. In most cases, this is quite true, but you'll see that some simple musical ideas can unlock the key to songwriting.
All music follows the basics of music. In the case of song writing, you will want to think through chord progressions that mainly follow the rules of the major scale. Those 7 notes--do re mi fa so la ti do---are the basis for all progressions and melodies. In other words, once you learn a chord family, you will be able to write all of the songs you want to in that key. Each chord in a chord family sounds good together, so you can't really go wrong in writing a song. There are many trends in song writing, which I cover in some of the member's only weekly updates, but for the most part you can experiment right away with song written once you learn a chord family.
The Guitarmann Essentals series is based off of the fundamentals of chord families. In other words, you will learn each of the 5 main chords families used with guitar. From there you will have the tools you need to literally write hundreds of songs.
If you are new to song writing, it's probably a good idea to first write out your chord progressions and then try to come up with some melody lines that fit in. Try to make the verse a little bit softer, then make the chorus louder and more exciting. The best teacher is experience, so try out as many things as you can, and soon you will find the magic chord progression and melody line that you are looking for. And you might even come up with the next big hit song!