Cassandra is a partitioned row store, where rows are organized into tables with a required primary key.
The first component of a table's primary key is the partition key; within a partition, rows are clustered by the remaining columns of the PK. Other columns may be indexed independent of the PK.
This allows pervasive denormalization to "pre-build" resultsets at update time, rather than doing expensive joins across the cluster.
DataStax has a free self-paced online course DS220: Data Modeling with Apache Cassandra.
A blog post by Sebastian Estevez describing a web based tool he created to help visualize data models (pre-3.0) as well as gives a tailored cassandra-stress configuration file to test the model. Using the Cassandra Data Modeler to Stress and Size Cassandra Instances
Patrick McFadin's data modeling series: