void comboBox1_SelectedIndexChanged(object sender, EventArgs e){
l1.Text = comboBox1.SelectedItem.ToString();
}
Or you can use SelectedIndex, which will be a zero based index.
Is this a homework question? If so, you would be much better off actually learning the material instead of posting it here. You will find your course/job/employment prospects very hard if you are stuck at this basic level and don't have the initiative to learn it yourself.