Place the cursor where you want the line
Type three hyphens and press Enter to insert a horizontal line
Type three underscores and press Enter to insert a thicker horizontal line
Type three equal signs and press Enter to insert a double line
Go to the Insert tab
Select Shapes
Choose a line shape
Click and drag to draw the line
Go to the Home tab
Select Borders
Choose Bottom Border to add a line
Go to the Insert tab
Select Horizontal Line if available
Use the keyboard shortcut for your version of Word if supported