Develop Word plug-in

Hi,

As I'm completely new to the Office topic I'd like to have back your possible guidance on the following issues.

I must develop an application - I suppose it will be kind of plug-in for a standard Word or I could wrap the Word called from my C# application - which is supposed to work in the follwoing way: I write a text under Word, I highlight a word, rightclick on it, context menu, choose an entry and trigger a processing. Word would serve just as text editor for document creation, storage, usual stuff ... The key here is the processing of the text I'll choose; in fact I want to store that word together with a property I've selected from the context menu, in some kind of repository (database, XML).

I'm not expecting how to get back the coded application from you ;-)!, just some hints where I'd have to look for so that to start getting into C# programming for Windows Office. Oh, last info, I'm using Visual Studio 2008 Pro (need any additional SDK or plug-in?).

Many thanks!