First you stop and think, "Hmmm... what music did have an impact on me, to make me think I could ever write a decent song? Hmmm." Then you find it on youtube. Then you post it here. And then you say something about it.
It can also be something that struck your fancy as recent as an hour ago. But nothing within the past hour. No wait, you can include that too.
Simple melody. And that simple melody is allowed to be shaded differently by different chords. Chords of a very basic and diatonic nature. I'm not saying it's the only way. But it is what stopped me in my tracks as a kid. It is what I miss most about a lot of new music. It is what I love to try and do with my music. Here are some examples of what I'm talking about. Not the arrangements. Not the groove, not the haircut, the era, the singer or any of the trappings.
Take any one of the songs below and learn to play the melody in either C or Am. Hello? Use a capo. Then play the chords and try to play the melody at the same time. All 1st position/Kumbaya chords. All the little suspensions of your chord to somehow get that melody note along with your fingered chord work. C Dm Em F G Am... C. This is what I did as a kid. I still do it all the time. And it lets me see and internalize the effect a note can have under the influence of different chords from the basic series. And in what order. And how do they build and add tension then release then fake out a release then build then... make you feel sad, then angry, then in a hurry, or lazy...
Just the melody and chords.