Start very very simple. Firstly choose a key, in general this is personal preference for you. So when starting off I'd say find a song you want to make something similar too, or a similar mood/feel and use that key. Start with a bassline, use the notes of the key until you have a simple bassline that sounds good, and by bassline I literally mean the notes, you can play with rhythm later on. When you have the bass, copy this to a saw for the chords and build the chords around the bass until it sounds good.
In general though the main thing is trial and error. Start simple and just keep playing til you find something good.
Hope this helps,
Harwood