How do I add products in cart after clicking add to cart button

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

public partial class Product_desc : System.Web.UI.Page
{
    SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["RegistrationConnectionString"].ConnectionString);
    int id;
   // String product_name, product_detail;
    protected void Page_Load(object sender, EventArgs e)
    {
        if (Request.QueryString["product_Id"] == null)
        {
            Response.Redirect("gold.aspx");
        }

        else
        {

            id = Convert.ToInt32(Request.QueryString["product_Id"].ToString());
            con.Open();
            SqlCommand com = con.CreateCommand();
            com.CommandType = CommandType.Text;
            com.CommandText = "select * from product_master where product_Id=" + id + " ";
            com.ExecuteNonQuery();
            DataTable dt = new DataTable();
            SqlDataAdapter da = new SqlDataAdapter(com);
            da.Fill(dt);
            d1.DataSource = dt;
            d1.DataBind();
            con.Close();
        }
        }

        protected void Button1_Click(object sender, EventArgs e)
    {
        //add to cart code
        
    }
}