Hey there Dermot, yes technically one can write a song on just about any instrument. If you’ve heard Janis Joplin’s Mercedes Benz then you can see what I mean. When it comes to writing for the guitar there are so many avenues to choose from. For me I like to start with either a really good melody that I play usually on a single string or two, or I start with a chord progression I really like. After developing a melody I try to base the song around the natural groove and rhythm of the melody or chord change. It can really be as simple or complex as you like, but the main thing is that you try to express what you’re hearing in your head. Remember, it all starts with what you hear, then you build.