From the menu, create a new Silverlight application

Make sure WCF RIA Services is enabled.

The result is a new project divided into two parts as can be seen in the Solution Explorer.

A first part called ObservableCollection and a second with the same name but with the extension .Web.

The first part can be seen as the Silverlight part running on your computer. The second one runs on the Internet. Communication between these two parts is done using WCF RIA Services.