You need to format the EBS volume (block device) with a file system between step 1 and step 2. So the entire process with your sample mount point is:
Create EBS volume.
Attach EBS volume to /dev/sdf (EC2's external name for this particular device number).
Format file system /dev/xvdf (Ubuntu's internal name for this particular device number):
sudo mkfs.ext4 /dev/xvdf
Mount file system (with update to /etc/fstab so it stays mounted on reboot):