To add line numbers using Microsoft Word 2007:

1. Click Page Layout at the top of the document.
2. Go to Line Numbers and select Continuous.
    

To add line numbers using older versions of Microsoft Word: 

1. Go to Format > Document.
2. Click the tab called Layout.
3. Under “Apply to:” select Whole document.
4. Click the button at the bottom that says Line Numbers
5. In the next screen, check the box that says Add line numbering
6. Use the following settings:

   Start at: 1
   From text: Auto
   Count by: 1
    

7. Under Numbering, check Continuous.
8. Click OK.
9. Click OK again.