How to write a song.

One way to do it is to close your eyes think real hard about who or what you are writing the song about.

Is it going to be a sad or dark song? Then take a look at this (minor scales).

Is it going to be a happy, light hearted song? Then think about writing it in a major scale.

Now I'm assuming that you aren't interested in a full-on music theory lesson so I'm going to put it like this:

First think about a tune. Doesn't matter if it's already taken by someone in a song. Now, think about adjectives that describe your song's subject.

Try and fit that tune into a scale. You can always use adjacent or complementary notes if it doesn't exactly fit into  it...and if it really can't be changed start playing with the tune until you get something that sounds okay, the best way to do it is to use metamorphosis, (meaning you just change it one note at a time ) 

If you play an instrument you might wanna think about adding some pentatonic licks to your composition. And why not go even further? Develop your song by learning the diatonic scales and modes nothing easier, as there are only seven of them :
Ionian, Dorian, Phrygian, Lydian, Mixolydian, Aeolian and the dreaded Locrian.

Now that you have the modes covered why not go even further than even further learn all about cadences and  how to use them in different contexts, enter the wonderful world of enharmonic relations and develop your music vocabulary by learning the chinese, Indian, Arabic,Romanian,Hungarian,Bulgarian,Slavic,Islamic and African(aka pentatonic minor) scales!!!
All in all this shouldn't take you more than 9-12 months to learn!

If this is too complicated for you than read through our next method:  

Learn four first position chords preferably major, do not make the mistake of leaning a 7th chord.You will never use it
Come out with some lyrics about 

a) Getting drunk and partying

b) Impossible love 

c) Impossible drunkness

d) Loving someone but not being able to confess because you are too drunk

e) Learning the Alphabet

Now mix the shit out of those 4 chords you learnt until you find a good combo.Don't bother about the rhythm...make it as basic as you can 

Either way you do it you can now enjoy a very successful musical career. 

If you followed the 2nd method then expect to get a massive amount of fans and money!!!!

I hope this helped.