Place the cursor where you want the line
Type three hyphens and press Enter for a straight line
Type three underscores and press Enter for a thicker line
Type three equal signs and press Enter for a double line
Type three asterisks and press Enter for a dotted line
Go to Insert
Select Shapes
Choose the Line tool
Click and drag to draw the line
Go to Home
Select Borders
Choose Bottom Border to add a line
Go to Insert
Select SmartArt or Icons if you need a styled line
Use the Draw tab if available
Select a pen tool and draw a line manually