Q:

how to return a cell into a variable in sql functions

I want to define a scaler function which in that I'm going to return the result into a variable but I do not know how to do this.
CREATE FUNCTION dbo.Funname ( @param int )
RETURNS INT
AS
declare @returnvar int
select  @returnvar = select colname from tablename where someconditions = something
return(@returnvar)

I want to make a function something like the top. I mean the result of the select statement which is:
select colname from tablename where someconditions = something

Is only a single cell and we are sure about it. I want to store it into a variable and return it from the function. How can I implement this thing?

A:

I should probably mention that scalar UDFs do come with a considerable health warning and can cause performance issues depending upon how you use them.
Here's an example though.
CREATE FUNCTION dbo.Funname ( @param INT )
RETURNS INT
WITH RETURNS NULL ON NULL INPUT
AS
BEGIN
RETURN (SELECT number FROM master.dbo.spt_values WHERE number < @param)
END

In the above example I didn't use a variable as it is redundant. The version with variable is
BEGIN
DECLARE @Result int
SET @Result = (SELECT number FROM master.dbo.spt_values WHERE number < @param)
RETURN @Result
END

For both of the above you would need to be sure the Query returned at most one row to avoid an error at runtime. For example
select dbo.Funname(-1) Returns -32768
select dbo.Funname(0) Returns error "Subquery returned more than 1 value."
An alternative syntax would be 
BEGIN
DECLARE @Result int
SELECT @Result = number FROM master.dbo.spt_values WHERE number < @param
RETURN @Result
END

This would no longer raise the error if the subquery returned more than one value but you would just end up with an arbitrary result with no warning - which is worse.
Following Comments I think this is what you need
CREATE FUNCTION dbo.getcustgrade(@custid CHAR(200)) 
RETURNS INT 
WITH RETURNS NULL ON NULL INPUT
AS
BEGIN
        RETURN
        ( SELECT [cust grade]
        FROM    ( SELECT  customerid,
                         DENSE_RANK() OVER (ORDER BY COUNT(*) DESC) AS [cust grade]
                FROM     Orders
                GROUP BY CustomerID
                )
                d
        WHERE   customerid = @custid
        )
END

