how to get the weekly sales of summing up each week in sql

hi 

i want to get weekly sales after summing up totalsales of each week. following is my table how to accomplish this task

CREATETABLE[dbo].[CashSales](
[CSNo][nvarchar](255)NOTNULL,
[CashDate][datetime]NULL,[CashCode][int]NULL,[CashName][nvarchar](255)NULL,[NetTotal][float]NULL
GO

NOTE: NetTotal column is net sales of every invoice

the result should be as 

1st week-may-2014  2nd week-may-2014  3rd week-may-2014

22422.00224243.00 264224.00