Hello Milan,
simply have a look at RfcStructureMetaData, RfcFieldMetaData, RfcTableMetaData, RfcParameterMetaData, and RfcFunctionMetaData. The first one is there to create structures, the second one for tables, and the last one for the function modules. With RfcStructureMetaData.AddField(RfcFieldMetaData) you can add fields to structure, with Rfc.FunctionMetaData.AddParameter(RfcParamaterMetaData) parameters to function modules. Read the API documentation for those classes to understand better how this is supposed to be used.
Best regards,
Markus