The best way to use fonts in Godot (2.1 or 3.0, at the time of writing) is to use DynamicFont. You can do so by selecting a node property which expects a font resource, clicking New DynamicFont in the dropdown menu, then loading a TrueType (
.ttf) or OpenType (
.otf) font file in the Font Data property. See the screenshot below (which is from the Inspector, while editing the DynamicFont resource):
Note that DynamicFonts are usually more suited to modern visuals (which rarely require soft shadows or gradients), and can easily be re-generated to several sizes (which is not the case of BitmapFonts).