Download a File Manager such as Filezilla (which I see you have already done).
The username and password to connect are not the same as the admin password. With your host should come a control panel such as cPanel which allows you to add FTP users, however most hosts have a File Manager withing the control panel which will allow you to maintain files.
Else you can download File Management extension from JED which will act as an FTP in the Joomla backend.
Hope this helps