You can either right click on the cell and select "format
cell" or go to Format > Cells from the menu bar. In the
format screen you'll see a tab marked "Border", from there
you can select to border the inside/outside of the cell,
or select which 'wall' of the cell you want to border by
clicking on it. Select a border style from the right side
then click on the 'wall' to change the style.
Additionally, there is a Border button on the formatting
toolbar which can do the same thing.
HTH