hello ayush,
just work on selected index change event of dropdown list.
protected void ddl1_SelectedIndexChanged(object sender, EventArgs e)
{
textbox1.Text=ddl1.SelectedItem.ToString();
textbox1.Text=ddl1.SelectedValue.ToString();
}
this type of answer you can find easily so before posting a question you should go for search.
Hope this will help you.
And don't forget to mark as answer if it helps. :)