If you mean a full line across the page, take a look at the solution to this question: Using Word, how can I create a document that looks like this?
- Select "Borders and Shading..." from the dropdown in the ribbon ("Paragraph" pane in the "Home" tab)
- Select the top and bottom borders on the right pane of the Borders tab
- Optionally, for spacing between the border and the text, select "Options..."
- In the dialog, enter the spacing that you want