how to store value in session

// storing value in session
string firstName = "Jeff";
Session["FirstName"] = firstName;
//reading value from session
string firstName = (string)(Session["FirstName"]);

Assign value to Session after successful login

Session["UserID"] = value;

here value can be anything like static or variable from Database....:)

like

Session["UserID"] = 10;

or

Session["UserID"] = Convert.toint32(DataTable1.Rows[0]["UserID"]);

Protected btnLogin_Click ()
{
//Write you logic here and verify user ID and Password.
if(Valid user)
{
Session["UserID"]= txtUserID.Text.Trim(); //txtUserID is your user id textbox id
}
else
{
Response.Redirect("~/Login.aspx",False);
}
}