How do you make up a song?

Like what do you put and what do you gotta do??How do you make up a song?
if you mean the music composing , i usually grab my guitar or play piano when i'm feels like playing them , like when i'm mad or i'm happy or i'm sad i feel like playing one of my instruments to reveal my self and that's how i get the idea of my song , if you wanna turn it into a professional music it's all about experiment and education and how well you can work with music production softwares , if i wanna make music for someone (for money of course!) i try to make a melody in my mind the i play it on a instrument and then i work on it to make it a professional song which of course it won't be as beautiful as the song i made by my emotions. any way i don't think it's something you can learn just by telling , if you want to make a song first of all you have to learn an instrument then you have to try and experience and try and experience till you find out what you gotta do . i think if you see the musical construction of one of your favorite musics it helps you learn what's what. try to find the midi files of your favorite song and try to concentrate on the drum loops , bass lines , guitar chords , lead methods, and..... good luck ;)