You need a
Font resource, which you can
load() if you already have one (recommended), or create dynamically.
How you load a font depends on whether you want to use DynamicFont or BitmapFont. The latter is a little easier, especially if you already have a ".fnt" file. I recommend reading through those docs to see how it all works.
It's probably easiest to use the editor to add the font to a node (such as Label) and then save the resource. Once you have a saved resource, you can reuse it anywhere you want.