The IBroker and IService interfaces are where you want to start.http://www.rightedgesystems.com/documentation/developerguide/html/AllMembers_T_RightEdge_Common_IBroker.htmhttp://www.rightedgesystems.com/documentation/developerguide/html/AllMembers_T_RightEdge_Common_IService.htm
The service interface is what "describes" the service. I.e. it's a broker service, it's name is xyz service, it does/does not have settings associated with it, etc.
The IBroker implementation does the work. It's the bridge between RightEdge and your broker. Since broker services are typically complicated and highly specific to the broker, we were unable to come up with real world example to ship with the product. However, I think it is valuable to view the example that we did ship (in the samples folder) because it's highly simplified and will get you acclimated with how the pieces fit.
Once you're comfortable with that, we have released the source to our Interactive Brokers plugin. This will give you an idea of how a real world implementation would be done.
The IB plugin source can be found at http://www.rightedgesystems.com/forums/Topic8819-16-1.aspx
Finally, there are many active participants on our forums that have implemented plugins of all types, including broker plugins. It would probably be a good idea to post questions regarding your plugin there.