Code:

private void Init()
        {
            List<string> brands = new List<string>();
            List<string> cars = new List<string>();

            brands.Add("audi");
            brands.Add("nissan");
            brands.Add("zastava");

            cars.Add("Q7");
            cars.Add("Skyline");
            cars.Add("Kec");
            
            ddlFirst.DataSource = brands;
            ddlSecond.DataSource = cars;           
        }

        private void ddlFirst_SelectedIndexChanged(object sender, EventArgs e)
        {
            try
            {
                switch (((ComboBox)sender).SelectedItem.ToString())
                {
                    case "audi":
                        ddlSecond.SelectedIndex = 0;
                        break;
                    case "nissan":
                        ddlSecond.SelectedIndex = 1;
                        break;
                    case "zastava":
                        ddlSecond.SelectedIndex = 2;
                        break;
                }
            }
            catch(Exception)
            {
            }
        }