There are a couple of ways to do as you wish. Both methods require that you select the cell, or cells, that you wish to 'border'.
You can then use the default Border tool, which will apply the border using the default color (usually black).
The Border tool is in the Font group in the Home tab. The icon is a small grid with a dark border across the bottom, right under the word Font. Click the drop down and select the border you wish to apply to the selected cells.
The more flexible method is to right click any of the selected cells and select 'Format cells'. Click the Border menu item when the Format window opens.
Select the Style of border you wish and the color to be used for the border. As the default color (black) can be overpowering at times, you can often use a softer color to add the grid lines but be more in the background.
Click the Outline and Inside options at the top and borders will be applied to all cells selected. Of course, there are a number of other bordering options you can select, also.