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One value-add you get with DataStax Community (versus the vanilla Apache Cassandra bits) is the OpsCenter utility. Bring it up on your machine by firing up your browser (as long as it's not Internet Explorer, which is unsupported) and navigating to http://localhost:8888.
Look around the dashboard and then, when you're ready to create a database, click the "DATA MODELING" link on the left nav bar (highlighted).
Keyspaces are the units of replication in Cassandra, but you can also think of them as the rough equivalent of databases. The Data Modeling screen lists all existing keyspaces and provides an "Add Keyspace" button (highlighted) to add a new one. Click it now.
The Add a Keyspace dialog should now pop up. Enter a name for for your new keyspace, then click the "I would like to create a column family" checkbox (a column family is Cassandra's rough equivalent of a table) and click the "Save Keyspace" button.