DocumentDB will compete with open source alternatives that have already gathered significant momentum in the market such as MongoDB and CouchDB. One of the key differentiators reported so far is improved support for high availability, transactions and scale out than MongoDB or CouchDB.
Purchasing of DocumentDB as a service will be done through “capacity units” that reserve high performance storage and dedicated performance. Note that this is quite a different model than the existing Azure Blob Storage and Table Storage models where you always buy a shared service and you only pay per storage unit and not for dedicated performance.
Provisioning a DocumentDB of your own is of course done through the Azure Portal under the Data, storage, cache and backup category.
You can find some starting tutorials here.