how to develop modular software with web services

I need to develop this application which:

Has 1 central module, and 3 sub-modules. In a simple java project would be:

package com.proj.manager
   Gui.Java
   MonitorizeSubModules.Java
   WebServiceSOAP.Java
package com.proj.submodule1
   ManageConnections.Java
   DoThingsInThreads.Java
package com.proj.submodule2
   ManageConnections.Java
   DoThingsInThreads.Java

Because each one of these packages will tend to be complex, I would like to develop them in a modular way (for example, using OSGi), and provide internal APIs for them to communicate with each other.

So, my main question is: How do I develop my project as a modular application and how can I make the application interface (ex. com.proj.manager.WebServicesSOAP.Java) available as Web Services?

I hope I made my question clear, and that it is not too newbie for the community :)

Thanks.