Once you opened Filezilla FTP client, you may enter your domain name(or Server IP) under Host.
Then you may enter the username and password the same as your cpanel username and password.
Under port, enter 21 and click Quickconnect.
You should be able to connect successfully, if not, you may refer below to ensure all settings are made correctly.
1.Open the site manager.
2.Click on New Site.
3.For Host, you may enter ftp.yourdomain.com or server IP.
4.For Port, you may enter 21.
5.For protocol, choose File Transfer Protocol.
6.For encryption, choose Only Use Plain FTP(insecure).
7.For logon type, choose Normal.
8.For user, use your FTP username or usually cPanel username.
9.Enter FTP or cpanel password.
10.Then, click on Connect.