I have started a Cassandra server using Bitnami AMI, I have attached volumes to the EC2 instance, but how would I set Cassandra to use the attached volume?
data_file_directories (use this link if you are on 3.0+) will tell Cassandra where to look for commitlog and data files (there are a few other directories you might be interested in, search through the documentation page for 'director'). However, your volumes have to be mounted first to be able to put any directories onto them.
How to mount them is not really a DB question, though. It is described in any case in the EBS documentation.