One important differance between MongoDB relational or SQL type databases you might be used to working with is that it is composed of Documents and Collections which are essically Schema-less.
Just because MongoDB is mostly schema-less doesn’t mean that there aren’t best practices and tips to properly store your data. These videos will help you get a grasp on how to structure your data.
Kyle Banker’s presentation at MongoNYC [slides]
Kyle Banker works at 10gen, where he maintains the MongoDB Ruby Driver and supports the Ruby developer community. Previously, Kyle built e-commerce and social networking applications at Alexander Interactive, and he once thrived as a languages nerd and a teacher of English lit.