Dialogue conventions do have some clear guidelines for paragraphing. During an exchange, for example, you generally start a new paragraph every time a different person speaks. Most paragraph breaks, however, aren’t as easy to determine.
In general, start a new paragraph when there’s a shift in focus, idea, or direction. Lorrie Moore’s novel A Gate at the Stairs opens with a student spending the week trudging through the winter cold from one interview for babysitting work to another. She thinks of the songbirds that lingered because of the late winter and how those birds disappeared by the end of the week. She wonders what happened to them, imagining them “in stunning heaps in some killing cornfield outside of town, or dropped from the sky in twos and threes for miles down along the Illinois state line.” The next paragraph shifts to the nature of her job search: “I was looking in December for work that would begin at the start of the January term.” It explores her tolerance for children and how suited she thinks she is for childcare. It’s a subtle shift—from the physical act of looking for work to her motivation for the work—and it’s a good place to start a new paragraph.
You can also start a new paragraph when there’s a shift in time and place. Lisa may be in bed, half awake at five in the morning as her husband prepares for a run. In this paragraph she may consider the strangeness of this behavior, since it’s not something he’s ever done in their seven year marriage. Then, the narrative might jump to a few minutes later, after her husband has left for his run and Lisa is in the kitchen preparing a cup of tea. That’s a perfect opportunity for a paragraph break.
Knowing when a shift is significant enough for a new paragraph is an acquired skill. When you read, pay close attention to the choices an author makes. Some shifts will be dramatic, while others will be subtle.
Don’t worry too much about this as you’re writing. When you go back and reread, you’re bound to find natural breaks.