To login to backend of Joomla site, type "administrator" after your domain name.

For example, if your site is called www.domain.com, to login to backend of the site, your domain should look like www.domain.com/administrator
Once you load the administrator page, you should see something like this:

Type in your username and password and press "Login".

 After successful login, you should see the main "Control Panel" like below:

You can add/edit/delete content of the site from here. That's it, your logged in!