how to get a selected column's data in string from Gridviw

Question

i am binding data in Gridview control in page load event 

how to get a selected column's data in a string from the Gridview control  

(ie)It is the primary key of that table,so for every row selected by user there will be a primary key ID column bounded to it..... so i need to get that ID & with this ID, i need to link with the next page of my web application

using Visual Studio 2005 asp.net C# 2.0