What do you want to compose in the song? The music? The lyrics? Or the whole thing?
For lyrics I think its practice and inspiration – you need to have a message – write it down, and usually since songs that rhyme sound better you’d want to find words in your message that go together. Its just practice makes perfect
For music lol you’d first need to learn an instrument… once you know how to play one thats when you can start to just play around with it and try to make your own tunes.
I do think its down to talent but I do believe anyone can do anything if they really really want to.