In order to access your cPanel (if you are a SiteGround customer) first try logging in your SiteGround Customer's area -> My accounts and click the "Go" button under cPanel's column for your account.
A new page with two possible ways to access the cPanel will load - via secure or normal connection. Note that in case your ISP(Internet Service Provider) filters the connection and blocks the standard secure cPanel port (2082 or 2083 ), you will have to use the normal connection.
Alternatively, no matter whether you are a SiteGround customer or not, you can access your cPanel directly through:
http://www.yourdomain.com:2083 (for secure connection)
http://www.yourdomain.com:2082 (for normal connection)
SiteGround offers the best cPanel hosting package. With it you can access and manage your cPanel without any problems.