While you're editing a page, you find the spot you want to add it, then put the section name in between equals signs on a new line, like this;
== Trivia ==
Then you add the new info below that.
The number of equals signs indicates the level/size of the section name. One = on both sides is a level 1 header. It's rarely used because it makes the text too large.
The example above is a level 2 header and is the most commonly-used for major sections. It's drawn with a line underneath it to separate it from the rest of the page.
Three (===) is a sub-header and the text is a little smaller. Four goes a little further and makes it a sub-sub-header with even smaller text.