Being designed to work with TVs, you may find that your Pi is only using the middle portion of the screen and there is a big unused area all round the screen.
This is not true of all TVs and monitors, but if it's happening for you then selecting the option to Disable Overscan may fix this for you.
Use the left and right cursor keys to make your selection and then hit Return.
This guide was first published on Dec 04, 2012. It was last
updated on Nov 16, 2018.
This page (Using the Whole Screen) was last updated on Feb 27, 2018.
If after disabling overscan, you cannot see the left edge of the screen then see the section 'Running Raspi_Config After Booting'.