Hi,
Take two controls in page. One is Textbox and another is Dropdown.
Set the css style for dropdon as below:
.DropDown{
display:none;
}
Use this javascript coding on click of textbox:
function fnShowDropDwn(){
var txt=document.getElementById("MyTextBox1");
txt.style.display='none';
var ddl=document.getElementById("MyDropDown1");
ddl.style.display='';
}
All the best.
--Amit