To mark the end of a paragraph in TiddlyWiki you need to type enter twice to create a double line break:

This is the first paragraph.

And this is the second paragraph.

Single line breaks are ignored within paragraphs. For example:

This is a
paragraph made
up of
short lines

<p>This is a
paragraph made
up of
short lines</p>

For situations where this behaviour isn't convenient, you can also use Hard Linebreaks in WikiText.