how to store this GridView data into database on button click....plz help me out

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;
using System.Web.UI.WebControls.WebParts;

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            datastructure();
        }

    }
    protected void GridView1_SelectedIndexChanged(object sender, EventArgs e)
    {
        
    }

    private void datastructure()
    {
        DataTable dt = new DataTable();
        dt.Columns.Add("ID");
        dt.Columns.Add("FIRST NAME");
        dt.Columns.Add("CITY");
        ViewState["vst"] = dt;
    }


    protected void Button1_Click(object sender, EventArgs e)
    {
        DataTable dt = (DataTable)ViewState["vst"];
        DataRow newrow = dt.NewRow();
        newrow["ID"] = TextBox1.Text;
        newrow["FIRST NAME"] = TextBox2.Text;
        newrow["CITY"] = TextBox3.Text;
        dt.Rows.Add(newrow);
        dt.AcceptChanges();
        GridView1.DataSource = dt;
        GridView1.DataBind();
    }
}