I hope I'm not going to offend anyone by saying I'm not altogether sure you can 'learn' composition. But if you're not happy with what you're doing - indeed for no particular reason, or all possible reasons, look at as much great art and great photography as you can. I think learning by osmosis is as helpful, or more helpful than learning or following any 'rules'.

Best of all, take a life drawing class, or possibly any drawing class, make yourself think about how to put something into empty space.