SAP xMII and OLAP Queries (SAP BW)

I am not familiar at all with OLAP queries and MDX syntax.

When I try to create a query on BW source, I receive only syntax errors, but with very limited exlanations (in SAP xMII as well as in SAP BW MDXTEST).

Does anyone know a good MDX tutorial that could help me to start ?

Thank you for help,

Best regards