first android service

I creating a small application that will basically use a background server to send data over HTTP. I dont know how to create services. Secondly there will be a couple of activities in my application. I want the activities to display a Context Menu when data becomes available. How can i do both. I have search for a while but the code i keep getting dose not seem to run on 1.6 api. How can i create the service and how can my activities listen to updates so that when a update is available they display a message.

NOTE: I do not need help on the HTTP part and the server part only creating the service and my activities listening to updates.

Kind Regards,开发者_JS百科

Give the Service docs a very good, thorough read.