Your FTP login details are the same as your cpanel username and password.
To access your site via FTP:
SITENAME.COM
cpanel username
cpanel password
In order to use FTP, you will need to have an FTP client on your computer.
List Of FTP Programs:
http://gm-innovations.com/accounts/knowledgebase.php?action=displayarticle&id=35
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'.
Helpful Links:
cPanel User Documentation
https://documentation.cpanel.net/display/ALD/cPanel+User+Documentation
cPanel Tutorials
https://www.youtube.com/playlist?list=PLZk46idJS6s54hAX8K79_AY8brgBwPtHd
cPanel & WHM Documentation (Dedicated Server Clients & Resellers)
https://documentation.cpanel.net
WebHost Manager - WHM User Guide (Dedicated Server Clients & Resellers)
https://documentation.cpanel.net/pages/viewpage.action?pageId=1507786