What are the different parts of a song?

What are the parts of a song? Obviously the lyrics, but what are the different parts of the actual music? And could you please explain each part? By part I mean like melody or harmony etc (I've only really heard of those, and dont really know what they are :). I want to write my own original music, and I want the genre to be pop, like similar to maroon 5. What part should I start writing first?Also, are there things that are usd commonly in pop music? Like I IV V progression, other things like that that are common in pop music? This would help me out so much! I'd appreciate it more than you would ever know, and thanks a bunch for your time!