Disable text box on the click of a radio button

[alobi]

My first posting was not clear enough. I am sorry, my bad. Here is a better posting. On my form, I have two option buttons, one for phone number and the other for last name. I have two text boxes , one for phone number the other for last name My goal is when the phone Number option is clicked to disable the last name text box, and enable the Phone Number text box, and when the option button last Name is clicked to disable the Phone Number text box and enable the txtlastName. Onformload. I need both options unchecked and the text boxes enabled

 function disablefield()
 {
 if ( document.admin.customerInfo_0.checked = true)
 document.admin.txtPhoneNumber.value = enabled
 elseif (admin.customerInfo_1.value="yes")
 document.admin.txtLastName.value.enabled=false
 }

<form action="post" method="get" name="admin">
<input type="radio" name="customerInfo" value="radio" id="customerInfo_0" />
<input type="radio" name="customerInfo" value="radio" id="customerInfo_1" />
<input name="txtPhoneNumber" type="text" id="PhoneNumber" size="25" />
 Last Name<input name="txtLastName" type="text" >