By default droplets do not provide an FTP service as this un-encrypted protocol is inherently insecure and not recommended for production use anymore.

Instead droplets do provide an SFTP service by default which can be accessed using the same information you use to log in via SSH. In a client like Filezilla your settings would be:

Server Name: [ip address]
Protocol: SFTP
Port: 22
Login type: Normal (unless you are using ssh keys instead of a password)
user: root
password: [your droplet's root password]