sql – Find the maximum average over a specific period, two tables

My task sounds like this: “Select sales territory (name) with sales in May 2013 higher than the average monthly sales per sales territory (Use SalesTerritory, SalesHeader tables).” As I understand it, logically, I need to find what territory was the maximum average for May 2013, while I need to link two tables (the “name” field in the “salesterritory” table, the rest of the data in the second, but the “name” must be present).
I tried to divide the task into parts, and find at least a territory by id without a name, here is my code:

SELECT TerritoryID, MAX(avga.sal)
from (select YEAR(OrderDate) AS 'Year', MONTH(OrderDate) AS 'Month', TerritoryID, AVG(TotalDue) AS 'sal'
FROM Sales.SalesOrderHeader 
GROUP BY YEAR(OrderDate), MONTH(OrderDate), TerritoryID 
having YEAR(OrderDate)=2013) as avga 
group by TerritoryID

This result does not appear to be correct even at this stage. Please help how to do it right? At least without the second table.