This post is a pretty good walk through on creating a custom plug in for EPiServer:

Creating an EPiServer Plugin: http://www.frederikvig.com/2009/10/creating-an-episerver-plugin/

Ps the link I mentioned is for EPiServer 5 but the instructions should still work in 6. However If want to save settings as described in the article I'd recommend going for the DDS. 

Thanks for the quick reply, however i dont think its a plugin i need.  Let me try to explain what i want to acheive,

EPiServer is the CMS for a Product website, each product has its "marketing" information held and edited in EPiServer.

ProductManager is another App that manages the products price, discounts etc.

It would be great to hold the Prodcuct Manage behind EpiServer login, and better still, be able to access EpiServer details to display in the Product Manager (Image, Name etc)

Is it still a plugin I need? or something else?

Thanks for the support...

If you want to fetch information from EPiServer you could you the web services. Check out http://world.episerver.com/Documentation/Items/Tech-Notes/EPiServer-CMS-6/EPiServer-CMS-60/Web-Services/ for an idea what you can do with them.