I think this is along the lines of what you're looking for. It appears that you want to see the orderid, the subtotal for each item in the order and the total amount for the order.
select o1.orderID, o1.subtotal, sum(o2.UnitPrice * o2.Quantity) as order_total from
(
select o.orderID, o.price * o.qty as subtotal
from product p inner join orderitem o on p.ProductID= o.productID
where o.orderID = @OrderId
)as o1
inner join orderitem o2 on o1.OrderID = o2.OrderID
group by o1.orderID, o1.subtotal