For a narrative; every time the plot changes, or a new issue is brought up.
E.g.
Benny listened as the steady humdrum of passing cars filled the air. He placed himself down on the smooth stone near the front garden gate, and sighed...[more lines here] It wasn't his fault his parents were fighting.
{New Paragraph}
Mum was in the bathroom, Dad at work. They never spent time together anymore. Not like they used to... etc. It seemed to go on forever.
{New Paragraph}
As the sun shone over the garden, Benny...
etc.
Also, there needs to be a new paragraph everytime you use dialogue.