There are seven basic chords called triads.

CEG     DFA     EGB     FAC     GBD     ACE   BDF (which isn't used much in basic composition)

Write a pattern of eight triads starting and ending on CEG and play it on a keyboard.

Choose a note in the first triad (CEG) and create a string of notes either:

by step up and down (steps),

or using the notes of the triad (jumps) 

or a rhythm on the triad note, until you reach a note in the next triad.

Then repeat the process for the next triad and note until you have a complete tune!!

Composing can be so easy really...

Writing a "good" tune needs more structure but that is for another day...

If anyone out there in the big wide world would like some secret tips about how to compose music...watch this space!!