Hi there @Seachel.
You can find the Optus X SMART user manual here.
If you open the settings app, you should the option Storage and USB.
Here, you can monitor the total and available space on your phone.
The X Smart is compatible with a microSD card. You can use a Micro SD card to store photos,videos, song and more. That should free up some space on your device.
You can pick one up fairly cheap from a electronics retailer, even supermarkets such as Woolworths or Coles.