To set a session:-
Session("SiteName") = "DotNetFunda";
//Here you directly assigning value
Session("SiteName") = txtGetSiteName.Text;
//Here you will get the value from textbox.
To get the session value:-
txtShowSite.Text = Session("SiteName");
Suppose you want to clear the session variable, then write
Session("SiteName") = null;
If you want to clear all the session variables from your application, then use
Session.Abandon();
In case of ASP.NET application by default the ASP.NET session state is enabled.