Assuming you have volume attached to the logical device on a running instance (e.g. /dev/sdb).

 # format block device
 mkfs.ext3 /dev/sdb
 # create a mount point
 mkdir /mnt/ebsvol
 mount /dev/sdb /mnt/ebsvol
 # did it work?
 df -h

To unmount the block device (in case you want to detach a volume):

 umount /mnt/ebsvol