Installing via apt-get is always a good idea, as you keep getting goodies with a simple sudo apt-get upgrade. Unfortunately, Cassandra is not in Ubuntu's repository, and my Data Science students require it (for their assignment, nonetheless). Here's a recipe to make it work:

1. Add the datastax repository to the apt-get repositories echo “deb http://debian.datastax.com/community stable main” | sudo tee -a /etc/apt/sources.list.d/cassandra.sources.list
2. Add their key for verification, curl -L http://debian.datastax.com/debian/repo_key | sudo apt-key add -
3. You need to sudo apt-get update to update the list of repositories to use the datastax one
4. From here it should be easy sudo apt-get install cassandra

Cassandra (currently 3.0.0) should automatically start (and run) on port 7000 (default).