How do you start writing a song?

First, you must create a short and concise sentence that describes what you want to COMMUNICATE in each section of the song. In addition, you should keep in mind that stanzas have to shape your great idea of a chorus. Your chorus should be the “I said all that to say this section of the song”. While it is true that there are many songs that use the same four-chord progression at all times, my personal recommendation is to write new chords for the verses.

So if you have doubts, remember that when writing songs, or any type of composition, to suck less, write more. It's a simple and reliable process that will help you write a song of virtually any genre over and over again, whatever level of experience you're at.