Creating a sum based on Order ID

Hello

I have a database that captures orders for my customers. I have one
form where I enter the orders and the various line items. I am
building a second form to record payments. On the second form I would
like to see a total that captures the sum of the order amount for all
the various line items related to a particular order #.

The formula Qty. * Price works well for the first line of detail but
does not pick up subsequent detail lines of the order (for example if
customer orders more than 1 item).

Could someone provide some guidance on how to create this sum. FYI, I
am not overly familiar VB)

Thanks in advance.

Joe