Solver for Inventory Control

[Rashdan]

Hi!
I've a problem, I have read few tutorials online for Excel Solver, but I
still don't know which columns to put in the solver. This is my first time
I'm using Solver.
I've two products ProductA and ProductB, and then I've columns for both the
products:

Inventory in store
Demand
Inventory outgoing
cost
if product to order
how much to order

Now, I want to know if I've to order a product which , in Inventory is Store
, is less than 6. But I can only order a product either 0, 10 or 20

How can I do that.. I'm really confused how to use it.
I've tried everything but nothing helped.
Please, help and let me know where to put what parameters.

Best Regards.
/Rashdan