Place the cursor where you want the line
Go to the Home tab
Click the Borders drop-down in the Paragraph group
Select Horizontal Line or Bottom Border
To insert a custom line, type three hyphens and press Enter
To insert a thicker line, type three underscores and press Enter
To insert a decorative line, go to Insert
Click Shapes
Choose Line
Draw the line where needed
Adjust the line length and thickness as needed