Well, it depends on how complicated you want to make it. Do you need confirmation of the Sales Order creation in "real time"? If so, you'll need to use a Web Service and ensure that there is a network path from wherever customers will create orders (public internet, extranet) to your NAV Web Service - likely using a VPN tunnel, etc.
es th
Alternatively, if you can live with a batch type process then you can have your customers create SOs via a web-based form, etc. and then import these orders into NAV on a regular basis using Dataports or XMLPorts.
For example, you may have a form online that your customer can create an Order on that places the Order in a staging table in SQL or even an XML or CSV file. Then you can run a process on a regular basis that imports these Orders into NAV and creates the appropriate SalesOrders.
Presumably, you also need a way to expose your Item database to the Ordering interface so customers can select which Items to order (and therefore create SalesLines from).
Which type of scenario are you interested in?