Set the identity property for Id in 1st table....
create procedure
dharani @FirstName varchar(20),@OrderDetails varchar(20),@LastName varchar(20),@OrderNumber int
as
begin
declare @Temp int
insert into Hcl.dbo.a values(@FirstName,@LastName);
select @Temp=Id from Hcl.dbo.a where FirstName=@FirstName
insert into Hcl.dbo.b values(@Temp,@OrderDetails,@OrderNumber)
end