how 2 reterive dta using drop down

Try This:
Retrive Data in Gridview ASP.NET c#

SqlConnection con=new SqlConnection("Data Source=AS1;Initial Catalog=practice;User      ID=as");
  con.Open();
  SqlCommand cmd = new SqlCommand();
  cmd.Connection = con;
  cmd.CommandText = "SELECT * FROM tbl_bio";
  SqlDataAdapter da = new SqlDataAdapter(cmd);
  DataSet ds = new DataSet();
  da.Fill(ds);
  GridView1.DataSource = ds;
  GridView1.DataBind();
  con.Close();

Please check this : http://www.w3schools.com/aspnet/aspnet_dbconnection.asp