For creating the logged in session ID, you need to take a string variable for that for example ::
string strSessionID;
strSessionID = HttpContext.Current.Session.SessionID;
Your sessionID will store in strSessionID.
and for creatring the session variables for displiying the data like firstname,last name on page/website, go through below format:
session["firstname"] = fistname;
session["lastnane"] = lastname;
Now you can use your above session variables on any your web pages.