CRM 2011 + webservice

Hello,

I am using CRM 2011 and new to it.

 I have an webservice(object)  which requires parameter ,the parameters will be the Enities properties which is created in CRM 2011.

like if I have created Enitiy 'order' which has fields like pickup date,weight,total amount

and these are parameters which will be required for webservice for Courier.

how would I fetch data from CRM 2011 and pass to webservice .

I known plug-in are used for that, but how to write? and how to access the database? 

please some one will explain me how to achieve this task?

all sort of Information are welcomed.

Thanks