Recommended:

Backup your site (both database & site files).

Minimum:

Backup your database.  If you are unsure how to do this, check with your host. Most shared hosts offer a database manager from their control panel.  phpMyAdmin is the most commonly used database manager. Some site control panels also offer a one-step database backup and download.  If you have cPanel on your server space, you will see the option to download a database backup.

Backup your configuration.php file.

Backup any custom files you are using on your site.
These may include your template directory and all its contents, your custom favicon, customised htaccess files, and any core files you may have modified.

Save the backed up files and database sql file to your local computer for safe-keeping.