Redirect user if not logged in using C# in ASP.Net

Hello Forum, 

Please how do I secure my web pages or web forms from users who have not registered?

For example I don’t want them to have access to other pages without registering. I sit that I will have to secure Session and redirect them to default page?

This is my session on page load event

    protected void Page_Load(object sender, EventArgs e)
    {
        showdata1();
    }
    public void showdata1()
    {
        SqlConnection con = new SqlConnection("Data Source=(LocalDB)\\MSSQLLocalDB;AttachDbFilename=|DataDirectory|\\Dataregister.mdf;Integrated Security=True");
        con.Open();
        SqlCommand cmd = new SqlCommand();
        cmd.CommandText = "SELECT * FROM UserTable WHERE email = '" + Session["user"] + "'";
        cmd.Connection = con;
        SqlDataAdapter sda = new SqlDataAdapter();
        DataSet ds = new DataSet();
        sda.SelectCommand = cmd;
        sda.Fill(ds, "detail");
        if (ds.Tables[0].Rows.Count > 0)
        {
            user.Text = ds.Tables[0].Rows[0]["email"].ToString();
        }
    }