You can create backups of your Pligg web site using the functionality described in the Pligg administration section. As an alternative you can use cPanel and FTP to create the backup. Note that you should save both the website files and the database for the installation.
The website files you can download to your local computer using an FTP Client. Do not forget that besides the directory where the actual web site resides you should also backup the website files directory you have setup during the installation. This is where all uploaded files are stored and as such it is an important part of your website.
The database can be backed up via the phpMyAdmin database management tool in cPanel.
Another alternative on SiteGround servers is to create a full backup of your account again via cPanel -> Backup functionality. It will include all website files and databases for your account.