I started writing songs as a by-product of playing the guitar. It just seemed like a natural progression to write songs, after I learned how to play songs, on the guitar. I just dabbled in songwriting (and didn't write many songs) during my early adult years. My writing didn't really take off until I went to Nashville (about 15-20 years ago) to celebrate a birthday; we went to the Opry and the Nashville Guitar Show (where I maxed out my credit card and bought a Martin guitar). For some reason, the purchase of that guitar motivated me to sit around and play a lot more, and write songs.
Initially, I just sat around the house, and played my songs in private; I rarely played them in public. Then, after my wife told me I was chicken for not playing my songs at the North Alabama Songwriter jam sessions (at the Coffee Tree)
Keep your guitars tuned!