This was answered long ago but there may be another answer, depending on the intent of the question.
If the question is - how do I change the font for every document I create in the future? Templates are your answer.
If the question is - how do I change the font used in this particular document? Read on...
Within a document, then default font is defined by a style.
You can see the style currently in use in the styles drawer - from the menu, select View - Show Styles Drawer.
(Your text is probably style Body or Freeform unless you have selected another style already.)
By default the style will use Helvetica - Apple seems to have bought Helvetica shares when they were cheap and now it uses it for everthing. My 17" Macbook Pro looks like Helvetica wrought in aluminium.
However, I can tell by your hat that you are more a Comic Sans type of person, so select some text and change it to Comic Sans.
In the style drawer, the current style (e.g. Body) now has a small red triangle next to it (some would say it is upside down, but I say that triangularity is independent of orientation,)
Click this red triangle to see an action list - choose Redefine Style... This sets the default font for the style to Comic Sans for every paragraph.
There does not seem to be any inheritance across the styles, so you will need to set the font for each style that you use. However, once done, it will apply to all usages of the style in the document.
