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
Go to Insert
Select Shapes
Choose a line shape
Click and drag to draw the line
Go to Home
Select Borders
Choose Bottom Border to add a line under text
Go to Insert
Select Table
Use a one-cell table and adjust the border to create a line