TAB key in the keyboard to go one field to another

In this code... when i run this page i want to use TAB key in the keyboard to go one field to another but work does not proper help me

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
public partial class Registration : System.Web.UI.Page
{
    SqlConnection cnn1 = new SqlConnection(@"Data Source=.\SQLEXPRESS;AttachDbFilename=E:\CMSWEB\App_Data\EMS.mdf;Integrated Security=True;User Instance=True");
       
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            FillCountry();
            FillState();
            FillCity();
        }
    }
    protected void FillCountry()
    {
        string strCmd="";
        strCmd = "Select * from tblCountry";
        SqlDataAdapter adpt1 = new SqlDataAdapter(strCmd, cnn1);
        DataSet ds1 = new DataSet();
        adpt1.Fill(ds1);


        ddlCountry.Items.Clear();
        if (ds1.Tables[0].Rows.Count > 0)
        {
            ddlCountry.DataSource = ds1;
            ddlCountry.DataTextField = "Name";
            ddlCountry.DataValueField = "Id";
            ddlCountry.DataBind();
           
        }
       ddlCountry.Items.Insert(0, new ListItem("Select", "-1"));
       
    }
    protected void FillState()
    {
        string strCmd;
        strCmd = "Select * from tblState where CountryId=" + ddlCountry.SelectedValue +" order by Name";
        SqlDataAdapter adpt1 = new SqlDataAdapter(strCmd, cnn1);
        DataSet ds1 = new DataSet();
        adpt1.Fill(ds1);


        ddlState.Items.Clear();
        if (ds1.Tables[0].Rows.Count > 0)
        {
            ddlState.DataSource = ds1;
            ddlState.DataTextField = "Name";
            ddlState.DataValueField = "Id";
            ddlState.DataBind();
           
        }
        ddlState.Items.Insert(0, new ListItem("Select", "-1"));
    }


    protected void FillCity()
    {
        string strCmd;
        strCmd = "Select * from tblCity where Stateid=" + ddlState.SelectedValue + " order by Name";
        SqlDataAdapter adpt1 = new SqlDataAdapter(strCmd, cnn1);
        DataSet ds1 = new DataSet();
        adpt1.Fill(ds1);


        ddlCity.Items.Clear();
        if (ds1.Tables[0].Rows.Count > 0)
        {
            ddlCity.DataSource = ds1;
            ddlCity.DataTextField = "Name";
            ddlCity.DataValueField = "Id";
            ddlCity.DataBind();


        }
        ddlCity.Items.Insert(0, new ListItem("Select", "-1"));
    }
    protected void ddlCountry_SelectedIndexChanged(object sender, EventArgs e)
    {
        FillState();
        FillCity();
        ddlCountry.Focus();
    }
    protected void ddlState_SelectedIndexChanged(object sender, EventArgs e)
    {
        FillCity();
        ddlState.Focus();
    }
    protected void btnSubmit_Click(object sender, EventArgs e)
    {
        string strCmd = "";
        int i = 0;
        strCmd = "Insert into tblUser(FirstName,LastName,Address,Email,Contact,Gender,Country,State,City,Password1,Notification,UserType) "+
                "values('" + txtFirstName.Text + "','" + txtLastName.Text + "','" + txtAddress.Text + "','" + txtEmail.Text + "','" + txtContact.Text + "'," + 
                radGender.SelectedValue + "," + ddlCountry.SelectedItem.Value + "," + ddlState.SelectedItem.Value + "," + 
                ddlCity.SelectedItem.Value + ",'" + txtPassword.Text + "','" + ChkNotification.Checked + "',"+radUType.SelectedValue + ")";
        SqlCommand cmd1 = new SqlCommand(strCmd,cnn1);
        cnn1.Open();
        i=cmd1.ExecuteNonQuery();
        if (i > 0)
        {
            lblMessage.Text = "You have successfully registered.";
        }
        else
        {
            lblMessage.Text = "Error during Registration.";
        }
        cnn1.Close();
    }
    protected void ddlCity_SelectedIndexChanged(object sender, EventArgs e)
    {
        if (ddlCity.SelectedIndex > 0)
        {
            ddlCity.Focus();
        }
    }
}