Before you can insert feature data into a datastore, you must apply a feature schema to it. That is, you must set up structures within the datastore to receive the data. The operation of applying a schema to a datastore is not provided by the Feature Service API. However, the operation of describing the existing schema or schemas in the datastore is provided in two ways:

MgFeatureService::DescribeSchema() returns a collection of MgFeatureSchema objects. See the Feature Schema overview.
MgFeatureService::DescribeSchemaAsXml() returns the schema collection in the form of an XML specification. There is also a convenient method for converting the XML specification of a schema collection into a collection of MgFeatureSchema objects, namely, MgFeatureService::XmlToSchema().