Composing your first song/music using ukulele is not suppose to be a daunting task that requires you to get a four year college degree.
It depends on how complex` or catchy (hook) you want your music/song to be. Sometimes those simple catchy tunes are the one that top the charts. Not to mention that once your creative juice starts flowing, Ukulele is a very handy instrument to write with, in comparison to using a piano or a guitar.
How to compose a song on ukulele? Composing a song on ukulele consists of having an idea for a singable melody, developing chord progression that fits your style, the lyrics for the song, a song form and finally the combination of your voice and your ukulele playing.
You could be riding a train or bus when that melodic, rhythmic, or harmonic idea hits you. The ukulele is your best friend. An instrument that would allow you to hear your melody and harmony (chords) right away.
Of course, a recording device like your smart phone can be very handy too so you would not loose that idea when inspiration hits you. For me, If you can hum a melody that’s already a good start. Otherwise, the ukulele can aid you to jumpstart either a melody, a chord, or simple rhythmic pattern, or a combination of all of them.