UDF That Runs A Dynamic Query

Good afternoon,
i'm new to Functions on the SQL server

I'm trying to create a dynamic query that would select the the column passed to the function from a certain table

my table called selected_Date, and has StartDate, and EndDate columns

when the user select for example "StartDate", i pass this as a variable to the function which runs the query. but i always gets back the passed string as a result..

here is my table

StartDate | EndDate
---------------------
20071231 | 20080306


here is my function knowing that i'm passing "Start" as a variable:


Code:

ALTER FUNCTION tu_efgn_int.Get_StartEndDates(@DateType varchar(10))
RETURNS varchar(8)
AS
BEGIN
DECLARE @vDate varchar(15)

Set @vDate= (Select @DateType + 'Date' From dbo.ps_tbl_SelectedDates)

RETURN @vDate
END



@vDate always returns "StartDate" as result instead of 20071231 why is that ?