How to change the default format in Numbers?
If by "underline a cell" you mean just a bottom border, this is done in the Cell tab of the inspector.


Select the cell(s) you wish to apply the bottom border to.
In the Cell tab on the Inspector select the border formatting.
There are several dropdowns that allow you to choose the style and weight of the bottom border.  See screenshots below.
Once you get your sheet the way you like it, save it as a template for future use.  (This can be your 'default')


If, however, you want the data or content only underlined, this is in the Text tab of the Inspector


Select the cell with your data
Apply the underline either by selecting it or using keyboard ⌘+U