Bottom line: The melody comes first, the chords come next, and the lyrics come last.  If you can hum or “na-na” a melody line and then put chords under it, you’ll have a better chance of producing a memorable song, period.

Start with the melody.  These come to me all the time.  When I’m driving with the radio off or I’m in an airplane or whatever.  It’ll usually be a melody that I’ll use for the chorus of a song.  I will “na na” the melody until it’s stuck in my head, then I’ll try to come up with a complimentary melody to use for the verses of the song.

Keep your chords basic.  Use primarily I, IV, V, and vi chords for a pop genre.  Try to match the key signature that you are humming.  Use the same progression for all the verses and a variation of the progression for your choruses.  Shoot for a 4 minute length.  When you’ve gotten accustomed to writing songs this way, you may want to add a “pre-chorus” melody & chords or a bridge.  However, avoid making your song any more complicated than it really needs to be.

Once you have your melody and chords worked out, you have essentially a “blank slate” for fitting the lyrics into.  Use metaphor.  Don’t be overly literal when you are writing lyrics.  The problem with literal lyrics is they don’t leave room for the listener to identify with your song in their own personal way.  Your song should be a piece of art - not a soap box.  Good art is good because people can identify with it.