I always have trouble finding the page for how to do formatting.
Here it is:
https://www.wikitree.com/wiki/Help:Editing_Tips
Items 4 and 8 will answer your question.
I was in the process of writing an answer, and looking, while you answered.
Kitty is right that <br> is useful and acceptable to create a line break.
However, <p> is not on the list of WikiTree recommended tags, and should not be used in biographies. To make a paragraph break, simply insert a completely blank line in the markup between the two paragraphs.
See
https://www.wikitree.com/wiki/Help:Recommended_Tags
https://www.wikitree.com/wiki/Help:Editing_Tips#Line_Breaks