What to back up
Your priority should be to back up your most important files as well as those that are difficult to recreate. For example, ranked from most important to least important:
This may include documents, spreadsheets, email, calendar appointments, financial data, family photos, or any other personal files that you would consider irreplaceable.
This includes changes you may have made to colors, backgrounds, screen resolution and mouse settings on your desktop. This also includes application preferences, such as settings for Writer, your music player, and your email program. These are replaceable, but may take a while to recreate.
Most people never change the system settings that are created when you first turn your computer on. If you do customize your system settings for some reason, then you may wish to back up these settings.
In general, you will want to back up files that are irreplaceable and files that require a great time investment to replace without a backup. If things are easy to replace, on the other hand, you may not want to use up disk space by having backups of them.