Your FTP login details are the same as your cpanel username and password.
To access your site via FTP:
In order to use FTP, you will need to have an FTP client on your computer.
List Of FTP Programs:
For further instructions: go to your cPanel, click on 'FTP Accounts'. Then next to your account under 'Special FTP Accounts' click on 'Configure FTP Client'.
cPanel User Documentation
cPanel & WHM Documentation (Dedicated Server Clients & Resellers)
WebHost Manager - WHM User Guide (Dedicated Server Clients & Resellers)