<asp:DropDownList ID="ddl" runat="server" onselectedindexchanged="ddl_SelectedIndexChanged" AutoPostBack="true">
</asp:DropDownList>
protected void ddl_SelectedIndexChanged(object sender, EventArgs e)
{
if(ddl.SelectedItem.Text="Yes")
{
txt_box1.Visible=true;
}
}
Note:-Autopostback="true" is compulsary,
Go to design view of ur form double click on it,you get the selected index changes code block in code behind