how to make next and previous row buttons work?

Question

In i have connected SQL 2008DB with asp.net 2010 web application using c# and I can not make next and previous buttons work,
and i want to show selected row number in label1 ,
only last and first buttons are working - Help me please
and i write these codes in defualt.aspx.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Windows.Forms;
using System.Data.SqlClient;
using System.Configuration;
using System.Data;
using System.IO;

namespace WebApplication1
{

public partial class _Default : System.Web.UI.Page
{
int i = 0;
DataTable dt = new DataTable();
DataSet ds = new DataSet();

protected void Page_Load(object sender, EventArgs e)
{
SqlConnection conn = new SqlConnection(@"Data Source=HARB\SQLEXPRESS;Initial Catalog=daily_log_golf;Integrated Security=True");
conn.Open();
SqlDataAdapter da =new SqlDataAdapter ("select * from school", conn);
da.Fill(ds, "school");
dt = ds.Tables["school"];
}
public void Display()
{
DataRow dr = dt.Rows[i];
TextBox1.Text = dr["id"].ToString();
TextBox2.Text = dr["name"].ToString();
TextBox3.Text = dr["email"].ToString();
TextBox4.Text = dr["town"].ToString();
}
protected void firstbtn_Click(object sender, EventArgs e)
{
i = 0;
Display();
}

protected void nextbtn_Click(object sender, EventArgs e)
{
if (i < ds.Tables[0].Rows.Count - 1)
{
i++;
Display();
}
else
{
}
}

protected void previousbtn_Click(object sender, EventArgs e)
{
i++;
if (i >= 0)
{
Display();
}
}

protected void lastbtn_Click(object sender , EventArgs e)
{
i = dt.Rows.Count - 1;
Display();
}

protected void showrowbtn_Click(object sender, EventArgs e)
{
//to show selected row number in label 1
DataRow row = dt.NewRow();
dt.Rows.Add(row);
// get the index now
Label1.Text = (dt.Rows.IndexOf(row)).ToString();
}

}
}