Indentation is used for:
* Paragraphs: The first line of a new paragraph is indented.
* Quotes: Longer quotes (usually four lines or more) are indented.
Topic sentences are typically:
* The first sentence of a paragraph. They introduce the main idea of the paragraph.
* Not indented.
Here's an example:
Incorrect:
> The topic sentence is about cats.
Correct:
> The topic sentence is about cats. This paragraph will discuss their playful nature, their love of naps, and their ability to be both cuddly and independent.