Script through geoprocessing service

I have a simple python script that calculates the slope of the raster (DTM).
I would like to publish this functionality as a geprocessing service on ArcGIS Server 10.1 Ent Adv.
Client will be an ArcObjects application.

The user should defines the area of ​​interest with polygon. This polygon is sent as a parameter to the geoprocessing service - there will be selected only a part of DTM defined with this polygon and in this area will be calculated SLOPE analysis. Result will be return to user (arcobjects application).

How to ensure that the polygon will be passed as a parameter?
How best to publish this functionality to ArcGIS Server (I have not experience)?

Thanks for the advice.

BR, RendyO