Modeling Graph Data in Cassandra DB
For Graph database on Cassandra have a look at TitanDB:

What you need is already implemented in TitanDB. Implementing your own Graph Database is not trivial, and would be very time consuming. In most cases, a proven solution is best. (BTW, I am not involved in TitanDB development or business.) I have no idea about your use case, but I do not see a reason to implement something new, except as a hobby.

Update I found a whitepaper about Titan GraphDB's data model in database: https://github.com/thinkaurelius/titan/wiki/Titan-Data-Model. It gives some hints how to design a datastore for graphs.

Aurelius is now also part of Datastax and they work on a combined solution for storing big graphs in Cassandra.