how to transform method into c # a method async c #?

Use Task.Factory.StartNew(Action t) that accepts an action that will execute it asynchronously:

public void cargar_datos() {

        System.Threading.Task.Task.Factory.StartNew(()=>{
                MySqlCommand cmd = new MySqlCommand("select id_zona, descripcion_zona from tb_zonas", conexion.obtenerConexion());
                MySqlDataAdapter da = new MySqlDataAdapter(cmd);
                DataTable dt = new DataTable();
                da.Fill(dt);
                //con.Close();
                conexion.obtenerConexion().Close();
                DataRow fila = dt.NewRow();
                fila["descripcion_zona"] = "selecciona una ubicacion";
                dt.Rows.InsertAt(fila, 0);

                combo_zona.ValueMember = "id_zona";
                combo_zona.DisplayMember = "descripcion_zona";
                combo_zona.DataSource = dt;
        });
    }