you need a matrix. You might want to use the Wizard to get the setup started. If you select Add New Report, in SSRS, it starts the Report Wizard and should walk you through most of it.

I have most of the report, and I did this:

Row Groups:

EmployeeID

CustomerID

Column Groups:

Order Year

order Month

Aggregate: SUM(LineTotal) -- I'm using AdventureWorks as a base.