To add line numbers to a word document go to Page layout > Page Setup > Line Numbers. I like to control the options so I select Line Numbering Options from the dropdown list.
On the Page Setup window that opens click on Line Numbers at the bottom right.
The select the options you would like. I find numbering every line distracting so I tend to set the Count by to 5, which only numbers every 5th line.
Click on ok and you are done.
And this is the final result.