You need to assign the returning value from the select to the variable you declared.

Like that :

DECLARE @var nvarchar(200)

select @var = top 1 field1 from table1

return @Var.

That will fix your problem, but I think you should use an Inline function for that, just do this :

create function fn1

@Param nvarchar(100)

returns nvarchar(200)

as

BEGIN

Return (Select top 1 field1 from table1 where Field2 = @Param)

END

Hope it helps, if you need some working code, just tell me, I only put sample for you to understand.