Developing an inventory management system integrated with Cash Register

Please, I need guidelines on the best and effective approach/method into developing an inventory and accounting management system that will handle transactions using cash register for medium size store.

The application will control the basic cash register operations like opening drawer, receipt printing and payments.

It's going to be a web application but should be able to control and respond com component operations such as drawer opening, receipt printing, barcode scanning etc. on the local computer system.

The 2 major tools I'm planning to use are C#.NET, WCF and JavaScript.

Thank you in anticipation of your help.

I am planning to commence development very soon, but a need the best design first.