Not knowing what your current knowledge level is I would start you out watching the Free Invoicing video.  I would actually point you to the members cut but I don't see that you are a member.

If you think about it adding and removing products is relatively similar to orders.  In addition check out the Inventory TechHelp.

With both of those videos there is a list in the description of videos that are kind of prerequisite.  Pretty close to the top of those lists would be having a good understanding of Relationships.

Check those out and come on back if you still need help.