Something like this (I used a cte to get a sort criteria added to the group by statement that is omitted in the final output)?

WITH cte as

(

SELECT datepart(mm,OrderDate) MonthNum,AVG(orderCount) AvgOrderCount, MIN(OrderDate) as m_date

FROM OrderTest

GROUP BY datepart(mm,OrderDate)

)

SELECT MonthNum,AvgOrderCount

FROM cte

Order BY m_date